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Hyperthyroidism in Cats (Felines): Causes, Symptoms, Treatments

Feline hyperthyroidism is characterized by overproduction of T4 (thyroxine) and T3 (triiodothyronine) hormones via the thyroid glands in cats.  As a result of excessive T4 and T3 hormone secretion, concentrations of T4 and T3 reach supraphysiological concentrations within the body of the cat.  At supraphysiological concentrations, these hormones elicit deleterious effects upon normative physiological processes, leading to disconcerting symptoms (e.g. appetite increase, weight loss, restlessness) – or even worse – death.

Preliminary reports of hyperthyroidism in cats surfaced in the 1970s, and ever since, the condition has garnered increased mainstream awareness and attention.  Although hyperthyroidism may be a standard part of the feline aging process, a staggering increase in the number of diagnoses since the 1970s has lead some to speculate that environmental toxins may be a causative factor.  While hyperthyroidism is understood to affect a small percentage of humans, particularly those with conditions such as Graves’ disease, it more commonly affects middle-aged and senior cats.

Currently, hyperthyroidism is regarded as the leading cause of endocrine dysfunction in cats.  Research indicates that approximately 1 out of 10 senior cats will develop hyperthyroidism and for many, it will be the chief cause of morbidity.  For this reason, it is important that cat owners remain cognizant of common symptoms and to consult a veterinarian for official diagnostic evaluation and treatment if feline hyperthyroidism is suspected.

What causes hyperthyroidism in cats? (Possibilities)

The cause of hyperthyroidism in cats may be subject to variation based on the particular cat.  One cat may end up with hyperthyroidism as a result of age-related thyroid dysfunction, a second cat may develop thyroid cancer that provokes the overactive thyroid, and a third cat may have been exposed to environmental toxins that alter thyroid function.  Moreover, it is necessary to consider that a combination of several factors (e.g. aging + environmental toxins) may act synergistically to cause hyperthyroidism.

Therefore, it is important to avoid generalizing that the cause of hyperthyroidism is the same for all cats.  Although a veterinarian may be able to speculate as to what the particular cause of hyperthyroidism may have been (e.g. a canned food diet), it is unclear as to whether these are accurate.  In any regard, below is a list of things that may cause hyperthyroidism in cats.

Aging: The average lifespan of a domesticated cat currently around 12-15 years which is over double what it was in the 1980s (of just 7 years) and still more than what it was in the mid-1990s (~9.5 years).  Due to the fact that life expectancy of domesticated cats has increased significantly since the 1970s, it is possible that in the past, cat owners never observed hyperthyroidism because the cat never lived long enough to develop the condition.  These days there is better medical care to keep domesticated cats alive for a longer duration, and as a result, perhaps hyperthyroidism is to be expected as a normal part of the aging process.

Most cats that develop hyperthyroidism are over the age of 10, and it wasn’t until the 2000s that the average life expectancy of domesticated cats reached 10.  It is likely that many (or most) cats contain genes the malfunction to cause hyperthyroidism once they reach a certain age.  These genes were likely passed on throughout generations because these genes were irrelevant to survival; cats didn’t need to live over 10 to pass on their genes.

Dietary intake: A cat’s diet could potentially cause them to experience thyroid dysfunction.  Not only do many cat-foods contain environmental toxins (PBDEs and BPA), but they may contain unsatisfactory amounts of iodine and/or soy.  Particularly, cats that either consume excessive or inadequate amounts of iodine tend to be at greater risk for hyperthyroidism.  Additionally, consumption of foods high in soy protein may cause thyroid dysfunction and ultimately hyperthyroidism in susceptible cats.

Iodine intake: There is reason to believe that either insufficient or surplus dietary intake of iodine may contribute to feline hyperthyroidism.  The average nutritional recommendations for feline iodine maintenance are around 50 mcg/kg per 24 hours (assuming healthy levels have already been attained).  Some research notes that few cats are actually getting enough iodine from their diets to maintain optimal thyroid function.

Short-term studies among have demonstrated that when iodine content within cat food is subject to variation, thyroid responses (as determined by thyroxine (FT4)) are highly variable.  Long-term studies highlight the fact that blood levels of thyroid hormone in cats are usually contingent upon iodine intake.  Researchers have noted that too much or too little iodine can cause hyperthyroidism in cats and can also lead to formation of thyroid goiter.

To really know whether your cat is getting enough iodine, a blood panel or professional consultation may be necessary.  Inadequate iodine intake over a long-term can clearly compromise thyroid health of cats.  Additionally, low iodine intake has been linked to development of thyroid nodules and thyroid adenoma.

Even cats that are seemingly receiving enough iodine based on food labels may not be absorbing it properly.  The exact bioavailability of iodine within cat food is unclear and may be subject to variation based on the specific source of food.  That said, cats regularly ingesting a nutritional surplus of iodine may benefit significantly from a reduction.

  • Source: http://www.ncbi.nlm.nih.gov/pubmed/12042516

Soy protein: In addition to iodine, it is necessary to consider dietary intake of soy protein.  A study published in 2004 documented the effects of a “soy diet” compared to a “soy-free diet” on thyroid concentrations in 18 healthy (euthyroid) cats.  The study was carried out over a term of 3 months and implemented a crossover design.

Results indicated that cats eating a “soy diet” exhibited significantly higher concentrations of T4 (thyroxine) and FT4 (free thyroxine), but concentrations of T3 (triiodothyronine) remained unchanged.  As a result, the ratio of T3 to FT4 was significantly reduced among cats consuming the “soy diet.”  Researchers concluded that inclusion of soy within diets has a modest effect on thyroid hormone concentrations in cats.

It is unclear as to whether inclusion of soy protein within the diet of cats can affect overall thyroid function.  Many speculate that soy disrupts the thyroid glands of cats and could potentially cause hyperthyroidism – especially when consumed regularly over a long-term; hence the increased T4 observed in short-term studies.  If you want to minimize likelihood that your cat develops hyperthyroidism, you may want to steer clear from products containing soy.

  • Source: http://www.ncbi.nlm.nih.gov/pubmed/15141877

Enlarged thyroid glands: Another potential cause of hyperthyroidism in cats is enlargement of thyroid glands.  It is unclear as to whether a prior factor (e.g. environmental toxins) lead to enlargement of thyroid glands, whether enlargement of thyroid glands occurred spontaneously to cause hyperthyroidism, or whether hyperthyroidism caused enlargement of thyroid glands.  In any regard, many cases of feline hyperthyroidism could be directly attributed to thyroid gland enlargement.

Among cats without hyperthyroidism, there is no apparent increase in size of thyroid glands when examined.  When hyperthyroidism occurs, most skilled veterinarians are able to pinpoint enlargement of thyroid glands with a thorough examination of the cat’s neck.  Enlargement is often caused by thyroid nodules or adenoma (benign tumors).

Environmental toxins: There is considerable evidence to link an increase of environmental toxins and the number of cats diagnosed with hyperthyroidism.  An upswing in the number of domesticated cats diagnosed with hyperthyroidism occurred in the late 1970s/early 1980s which happens to be around the same time industrial chemicals, particularly PBDEs, began polluting the environment. In addition to PBDEs being a prime causative suspect for many cases of feline hyperthyroidism, an increase in the usage of plasticizers containing BPA may also play a role.

PBDEs: Chemicals known as PBDEs (polybrominated diphenyl ethers) are compounds that are utilized most often as flame retardants.  PBDEs are found within a multitude of products such as: airplanes, building materials, furniture, electronics, foams, motor vehicles, plastics, textiles, etc.  Since the 1980s PBDEs became increasingly popular and widespread throughout the environment.

Cats are likely exposed to PBDEs from indoor dust, sewage, and foods (particularly those containing fish).  The problem with PBDEs is that they are highly lipophilic and resistant to degradation.  This means that they are likely to accumulate within the body of the cat and when high levels are reached, neurophysiology is altered – leading to hyperthyroidism and potentially more serious health problems.

BPA: The chemical known as BPA (bisphenol-A) is commonly utilized to make plastics due to the fact that it is clear and durable; it is included in consumer products such as: CDs, DVDs, food storage containers, water bottles, etc.  Upwards of 10 billion pounds of BPA are used on an annual basis throughout the world to manufacture polycarbonate plastic.  Ingestion of BPA is known to disrupt endocrine processes and exhibit hormonal properties.

It is considered harmful to fetuses, infants, young children, and has been linked to increased risk of cancer.  Since it is harmful to small humans, there’s a good chance that regular ingestion of BPA by cats (particularly small ones) will have deleterious effects on physiological processes such as thyroid function.  BPA appears to bind to thyroid hormone receptors and may alter its functions over time.

Thyroid nodules: Hyperthyroidism in cats is often caused by lumps or “nodules” that grow directly on or around the edge of the thyroid gland.  Nodules commonly appear in older cats and do not automatically indicate that the cat has hyperthyroidism, but they are often a sign.  Growth of nodules can lead to enlargement of the thyroid gland and/or overactivation, which in turn causes excessive secretion of T4 and T3 hormones – without being kept in check by the pituitary gland.

In most cases, nodules are commonly very small bumps, about the size of a “pea” and are felt as small bumps upon thorough examination of the neck and throat.  In rarer cases, nodules become large and can be observed with a brief visual inspection of the neck-area.  Understand that if your cat developed nodules on his/her thyroid, these could be a direct cause of an overactive thyroid.

Symptoms of Hyperthyroidism in Cats (Felines)

If your cat is suffering from hyperthyroidism or you suspect that your cat may have developed an overactive thyroid, it is necessary to beware of common symptoms.  Understand that the severity and number of symptoms experienced is subject to variation based on the particular cat.  Typically, the more severe the case of hyperthyroidism, the more obvious the symptoms will be to pet owners.

Assuming your cat is producing too much T4 (thyroxine) via the thyroid glands, an array of physiological processes will be affected.  Examples of physiological processes altered by hyperthyroidism include: digestion, heartbeat, kidney function, liver function, and nervous system.  Unless the hyperthyroidism is severe, most owners will notice symptoms emerge gradually and with time, these symptoms will become increasingly severe.

Aggression: Your cat may appear to behave more aggressively than usual – unpredictably attacking you, visitors, or other pets.  The aggression may be a byproduct of inner agitation and/or restlessness associated with hyperthyroidism.  The fact that excessive quantities of T4 and T3 are secreted by the thyroid gland gives your cat more energy than is necessary to properly function.

One way the cat may naturally cope with this excess energy is by attacking, hunting, and/or behaving aggressively.  The aggression is completely unconscious and a result of physiological disruption.  As you treat the hyperthyroidism, aggression should diminish and/or subside altogether – especially if your cat was never aggressive in the first place.

Apathy: If hyperthyroidism went undetected and untreated for a long-term, the health of your cat is likely to be poor.  Hyperthyroidism takes a toll on the cat both physically and neurologically, and after awhile, the cat may appear apathetic and unresponsive.  Don’t be surprised if your cat doesn’t want to play or eat normal portions of food if he/she is suffering from advanced-stage hyperthyroidism.

In some cases, cats may have a comorbid health condition (e.g. tumor) that is causing both thyroid dysfunction as well as neurological deficits.  Comorbidities and/or medications used to treat these conditions may lead your cat to appear apathetic.  That said, the majority of cats with hyperthyroidism will appear restless, jittery, and engage in odd behaviors.

Appetite increase: If your cat has an overactive thyroid, he/she may be eating more than usual.  When the thyroid glands secrete more T4 and T3 than necessary, it ramps up the cat’s basal metabolic rate (BMR) and more calories are burned at rest.  Due to the heightened metabolic rate resulting from hyperthyroidism, the cat will seek out extra food and/or appear more ravenous than usual.

You may observe your cat waiting for food even though he/she just finished a (seemingly) normal-sized or large meal.  You may also notice that your cat is waiting around for food longer than usual even after he/she just ate.  In other cases, cats may appear uninterested in food or have a reduced appetite as a result of general deterioration in health associated with severe / untreated hyperthyroidism.

Coat, Fur, Hair loss or thinning: Cats with hyperthyroidism often appear disheveled as if they’ve been living in the streets for the past few years.  While many health conditions can contribute to abnormal, unkempt appearance of cat hair – this is also a telltale symptom of hyperthyroidism.  Your cat may no longer engage in any sort of grooming or may groom excessively to the extent that their hair thins and/or appears ragged.

Changes in the appearance of cat hair are most obvious in breeds with naturally long-hair.  In some cases, the hair of your cat may appear dull in color, matted, or even unusually greasy in certain areas.  Certain cats may also have a noticeable rash (miliary dermatitis) that developed as a result of hair loss associated with excessive grooming behaviors (e.g. licking or scratching).

Diarrhea: It is understood that high levels of thyroid hormone can affect digestive processes and activate the intestines.  In cases of feline hyperthyroidism, food is shuttled through the digestive tract of the cat’s body at an unusually fast rate.  As a result, the cat may pass bowel movements more frequently than usual or appear to have loose stools.

If your cat has hyperthyroidism-induced diarrhea, you may find yourself cleaning out his/her litterbox more often than usual.  Additionally, you may notice that the stools are particularly bulky and/or malodorous.  If the diarrhea is ongoing and accompanied by other symptoms on this list, it could indicate that your cat has hyperthyroidism.

Rapid or difficulty breathing (Panting) Some cats with hyperthyroidism have a noticeably tough time breathing.  It may appear as if they have shortness of breath or are constantly gasping for air.  Difficulty breathing does not occur in all cats with hyperthyroidism and may only occur on an intermittent basis.  In other words, you may observe your cat breathing normally throughout the day, but suddenly appear as if he/she is panting or suffocating.

If the hyperthyroidism is severe and/or has been untreated for a long duration, there’s a chance that the difficulty breathing could be a sign of poor oxygenation.  Hyperthyroidism can take a cumulative toll on the functionality of the cat’s heart, ultimately decreasing oxygen intake with each breath.  For this reason, the cat may try to overcompensate by breathing more than usual in attempt to replenish oxygen stores.

Another reason that cats may appear to exhibit breathing abnormalities is a result of excess body heat.  Hyperthyroidism increases body temperature as a result of increased energy consumption and changes in blood flow, thereby causing the cat to become hotter than usual.  In attempt to cool themselves down, cats may engage in panting in attempt to reduce body temperature.

Excessive thirst: Heightened concentrations of thyroxine (T4) may lead your cat to drink more water than usual.  If you notice that your cat’s water dish is constantly empty and are constantly refilling it, it may be a sign that he/she is drinking more than usual due to hyperthyroidism.  Excessive thirst generally goes hand-in-hand with frequent urination – so expect your cat to urinate more often as a result of this increased water intake.

Fever: You may notice that your cat is running a low-grade fever as a result of hyperthyroidism.  While cats with hyperthyroidism do not usually have high fevers, they may experience an increase in overall body temperature.  Abnormally high concentrations of thyroid hormone tend to compromise thermoregulatory processes within the body of the cat and cause the body to generate more overall heat.

Your cat may appear to pant more than usual and/or seek out cool places in attempt to cope with a low grade fever or heat intolerance.  During a check-up with the veterinarian, the vet may report that your cat is running a mild fever.  A more thorough evaluation may reveal that the cause of this fever was hyperthyroidism.

Frequent urination: Many cats with hyperthyroidism will end up drinking a considerable amount of water – much more than usual.  As a result of increased water intake, expect more frequent urination.  You may observe your cat making more trips to the litterbox to urinate or find yourself cleaning out urine from the litterbox with greater frequency.

Hypertension: If your cat is dealing with hyperthyroidism, he/she may simultaneously experience high blood pressure or hypertension.  Cats with hyperthyroidism tend to exhibit systolic blood pressure over 190mm Hg even while relaxed.  Hypertension (as induced by hyperthyroidism) can become a major health problem for your cat by damaging blood vessels as a result of heightened pressure.

Damage to blood vessels from hypertension may lead to neurological deficits, vision problems (e.g. blindness, retinal detachment, etc.), hepatic impairment, and renal dysfunction.  If your cat is diagnosed with hypertension (high blood pressure), you may want to request a more thorough examination to rule out hyperthyroidism.  Usually when hyperthyroidism is properly treated, blood pressure normalizes.

Irregular or rapid heart rate: It is extremely common for cats with hyperthyroidism to exhibit an irregular or rapid heart rate.  In the average cat, a normal heart rate (while relaxed at home) falls within the range of 140 to 200 BPM (beats per minute).  If you take your cat to the veterinarian or your cat was recently exercising, his/her heart rate will likely exceed 200 BPM.

In cats with hyperthyroidism, heart rate could easily exceed 200 BPM while relaxed at home.  This is due to the fact that physiological processes are stimulated and expedited by heightened concentrations of thyroid hormone.  In addition to a rapid heart rate, you may notice irregularities in his/her heart beat; these are also caused by excessive secretion of thyroid hormones.

Some cat owners may report “heart murmurs” or palpitations characterized by loud, skipped, or irregular heart beating.  It may be difficult for you to tell what a normal heart rhythm is in a cat, so be sure to take your cat to the vet if you suspect any heart beat irregularities.

Nail growth, thickening, or breaking: You may notice that your cat’s nails grow excessively and/or appear thicker than usual.  They may also be brittle, prone to chipping, or break with increased frequency.  Significant changes in nails of cats are usually difficult to detect because they do not occur in all cats and most owners aren’t astutely observing their cat’s nails on a daily basis.  If changes in the density or growth of your cat’s nails are accompanied by other symptoms on this list, the nail alterations may be a result of hyperthyroidism.

Physical weakness: Although your cat may already be of old age and weak as a result of standard aging, hyperthyroidism can amplify physical weakness.  When the thyroid glands are producing too much thyroid hormone, basal metabolic rate increases and nutritional deficiencies are likely to occur.  As a result, cats may end up experiencing loss of bone and muscle tissue – making them weaker than usual.

Your cat may appear to have a tougher time moving around, sprinting, or jumping – due to weakening of his/her body.  Proper treatment and diet can reverse physical weakness stemming from hyperthyroidism.  If your cat appears extremely weak and shakes more than usual, it could be a sign of hyperthyroidism.

Restlessness: Felines with hyperthyroidism often appear restless, wired, and/or unable to sit still.  You may notice that your cat is unable to maintain calmness, is constantly on the move, and may act more aggressive than usual.  In cases of hyperthyroidism, the excess levels of thyroid hormones act to increase neurophysiological arousal – making it difficult for your cat to relax and just “chill.”

Your cat may appear as if he/she is hyped up on caffeine, or in more extreme cases, some sort of psychostimulant (e.g. Adderall).  As a result of this restlessness, you may perceive your cat as being uncooperative or having an attitude. Don’t be surprised if the excessive restlessness also interferes with your cat’s ability to get enough sleep.  With proper treatment for hyperthyroidism, your cat will be able to remain calm and appear less fidgety than in a hyperthyroid state.

Thyroid enlargement or nodules: Another common sign of hyperthyroidism in cats is the enlargement of the thyroid gland and/or appearance of nodules (bumps) around the thyroid gland located in the neck-area.  In euthyroid (normal thyroid) cats, the thyroid gland is typically unable to be seen or noticeably felt when the neck area is examined.  However, in most cases of hyperthyroidism, lumps develop on the thyroid or the thyroid appears swollen.

Keep in mind that not all cats with nodules or enlargement of the thyroid necessarily have hyperthyroidism.  That being said, if thyroid enlargement occurs (especially with a multitude of other symptoms), evaluation for hyperthyroidism by a veterinarian is warranted.  Most owners of will notice sight enlargement of the thyroid while petting the neck of their hyperthyroid cat.

Vomiting: Many cats with hyperthyroidism vomit frequently, leading owners to suspect that they may have consumed some rotten food and/or have another health condition.  If vomiting is accompanied by other symptoms on this list, there’s a good chance that your cat is dealing with hyperthyroidism.  Keep in mind that not all cats with hyperthyroidism will vomit, but in some cases vomiting can occur.

Some believe that the vomiting may be a result of gastrointestinal irritation associated with elevations in thyroxine (T4) concentrations.  Others speculate that the combination of gastrointestinal distress along with increased food consumption – provokes vomiting in a subset of hyperthyroid cats.  In any regard, if your cat is vomiting frequently and/or excessively – visiting a veterinarian is advised.

Weight loss: Despite an increased appetite, your cat will likely end up losing weight as a result of hyperthyroidism.  Hyperthyroidism speeds up the cat’s basal metabolic rate (BMR), leading the cat to burn more calories while resting.  The body of the cat is using up more energy than usual, and regardless of how much food is consumed, weight loss occurs.

The amount of weight your cat loses will likely be contingent upon the severity of hyperthyroidism and how long it went untreated.  Cats with severe hyperthyroidism that aren’t treated in a timely manner may lose a considerable amount of weight and appear sick.  Among cats with hyperthyroidism, weight loss is one of the most common symptoms.

While most of the weight loss will occur as a result of a faster-than-usual BMR, some may occur as a result of diarrhea, bone loss, and increased physical activity (i.e. exercise).  If your cat is losing weight, but his/her appetite hasn’t changed or has actually increased – consider hyperthyroidism as a potential cause.

Note: It is important to note that not all cats with hyperthyroidism will have every aforestated symptom.  Some cats may not have many noticeable symptoms, yet upon evaluation by a veterinarian, a diagnosis of hyperthyroidism is made.  Realize that symptoms tend to worsen gradually over time if hyperthyroidism remains undetected or untreated.

Diagnosis of Feline Hyperthyroidism

If you suspect that your cat has hyperthyroidism, it is necessary to consult a licensed veterinarian for a thorough medical evaluation.  With proper diagnostic testing, your veterinarian should be able to pinpoint the severity of your cat’s hyperthyroidism, as well as any comorbid medical abnormalities.  Diagnosis of feline hyperthyroidism is typically conducted via thyroid hormone panels, thyroid scintigraphy, and various blood chemistry panels.

Thyroid hormone panels: The simplest way to determine whether your cat is suffering from hyperthyroidism is to analyze his/her thyroid hormone levels with bloodwork.  This means that a small sample of blood will be collected from your cat and assessed in a laboratory to determine concentrations of circulating T4 (thyroxine) and T3 (triiodothyronine).  If concentrations of T4 and T3 are abnormally high, this indicates that the thyroid may be overactive and the cat could be suffering from hyperthyroidism.

However, since many exogenous factors (e.g. dietary intake, environmental toxins, etc.) and non-thyroidal medical conditions may affect thyroid hormone concentrations, thyroid hormone panels are not always accurate in diagnoses.  Some cats may exhibit abnormally high T4 and T3 according to a thyroid hormone panel, but further evaluation may reveal that the elevations were of non-thyroidal origin(s).  In most cases though, thyroid hormone panels should give the veterinarian an accurate understanding of whether hyperthyroidism is occurring.

It is also necessary to note that some cats with hyperthyroidism appear to have T4 (thyroxine) within a normal range on blood tests.  In early stages of hyperthyroidism, the thyroid gland may exhibit some degree of hyperfunction, but a blood test will be unable to detect it.  For this reason, it may be recommended to utilize a thyroid scintigraphy for a more in-depth understanding.

Thyroid scintigraphy:  This is a medical procedure that involves collecting a visual display of thyroid tissue based on the uptake of radionuclides.  Utilizing thyroid scintigraphy allows a veterinarian to collect anatomical and physiological data from the feline thyroid glands – pinpointing the exact location of tissue abnormalities.  In many cases, thyroid hormone panels (as evaluated from blood samples) are insufficient to determine whether a cat is truly suffering from an overactive thyroid; this is especially common in early stages of hyperthyroidism.

A thyroid scintigraphy allows veterinarians to diagnose hyperthyroidism prior to abnormal thyroid panels via bloodwork.  Another benefit of using thyroid scintigraphy is that it can exclude a suspected diagnosis of hyperthyroidism among cats with high thyroid levels stemming from non-thyroidal origins.  Thyroid scintigraphy allows veterinarians to elucidate the precise severity of hyperthyroidism in relation to other non-thyroidal medical conditions.

Additionally, thyroid scintigraphy gives veterinarians data regarding thyroid carcinoma (cancer of the thyroid gland).  While collection of a blood sample is a common modality by which hyperthyroidism is diagnosed, a scintigraphy is recommended for a more thorough understanding of the condition.  Analysis of a thyroid scintigraphy may be warranted prior to an actual blood test in cases of suspected hyperthyroidism or premorbid hyperthyroidism.

  • Source: http://www.ncbi.nlm.nih.gov/pubmed/16584025

Blood chemistry panels: Although a veterinarian will usually collect blood samples specifically to evaluate serum concentrations of thyroid hormones, additional blood testing is often conducted to evaluate your cat’s general health.  Many cats with hyperthyroidism suffer from comorbid health conditions and/or develop various medical conditions as induced by the overactive thyroid.  For this reason, is is usually necessary to evaluate cardiac, hepatic, and renal health of your cat with more extensive blood panels and a urinalysis.

Assuming your cat has hyperthyroidism, treatment for the condition often improves organ function, particularly of the heart.  Due to the fact that many signs of hyperthyroidism often are similar to those associated with other conditions such as: adenoma, cancer, hepatic impairment, renal impairment, etc. – a thorough evaluation of blood chemistry is usually recommended.  Other diseases can be ruled out and your veterinarian will have an easier time generating an accurate diagnosis.

Treatments of Hyperthyroidism in Cats

If your cat is diagnosed with hyperthyroidism, sufficient treatment is imperative to improve his/her overall health and functionality.  There are a variety of interventions that can be utilized for the treatment of feline hyperthyroidism.  Examples of some common interventions for the treatment of hyperthyroidism in cats include: radioactive iodine therapy, antithyroid medications (e.g. methimazole), dietary iodine restriction, and surgery.

Understand that each intervention for feline hyperthyroidism has advantages and drawbacks.  The intervention considered most advantageous with the fewest drawbacks is radioactive iodine therapy.  However, since radioactive iodine therapy is not optimal for all cats, other interventions should be discussed.  Usually the sooner a cat receives treatment for hyperthyroidism, the quicker his/her health will improve.

Radioactive Iodine Therapy

Perhaps the best treatment for feline hyperthyroidism is radioactive iodine therapy, a procedure involving administration of a single radioactive Iodine-131 injection.  The injection of Iodine-131 is delivered either intravenously or subcutaneously.  Upon injection of the Iodine-131, it travels directly to the cat’s thyroid gland and usually depletes overactive cells that use iodine to secret thyroid hormone.

In some cases, if the initial injection isn’t effective, a second injection may be required.  In most cases, a single dose will deplete a sufficient amount of overactive cells, while simultaneously leaving behind enough cells for your cat to manufacture enough thyroid hormone in the future.  Of the cats that receive radioiodine therapy, only a small percentage (~5%) will fail to derive sufficient benefit and require medicinal thyroid hormone post-procedure.

In the event that radioiodine treatment is considered “successful,” your cat will not require any additional thyroid treatment in the future.  Advantages of radioiodine therapy include the fact that it doesn’t provoke side effects, is painless, doesn’t require anesthesia, and is highly effective.  The only major drawback is that your cat will need to stay at a hospital for around 7 to 14 days post-procedure until radiation has abated.

It is important to note that radioactive iodine therapy is the most popular treatment for hyperthyroidism in humans.  Research suggests that cats treated with radioactive iodine therapy outlive cats treated with standard hyperthyroid medication (methimazole) by ~2 years.  This should be considered a first-line treatment for feline hyperthyroidism.

The only drawback is that radioactive iodine therapy may be considered costly for some cat owners at $800 to $2000.  When compared to the price of a medication like methimazole, many cat owners assume that ongoing medication is cheaper than one expensive procedure.  However, when considering prescription refills, veterinarian visits, and bloodwork required for medication – it usually makes more sense to pursue the radioactive iodine therapy.

Antithyroid Medications

A common way to treat feline hyperthyroidism is via the use of medications that aim to reduce thyroid concentrations.  Two of the most popular medications for the treatment of hyperthyroidism in cats include: methimazole and carbimazole.  The only major difficulty with medication is that they must be administered at a highly-precise dosage to be therapeutically effective.

Furthermore, each cat requires a specific dosage based on body mass and underlying severity of hyperthyroidism.  Administration of an insufficient dose will leave the cat stuck in a state of subclinical hyperthyroidism.  Administration of a dosage too high will suppress thyroid concentrations to an abnormally low extent – leading to an entirely new set of symptoms.

Usage of medications will require your cat to get regular bloodwork to assess whether concentrations of T4 (thyroxine) have reduced to a normative range.  Since many individuals dislike the idea of giving their cat a medication and making regular visits to the veterinarian for bloodwork (to determine thyroid concentrations and efficacy of the medication), other options may be preferred.  Additionally, some cats may require b.i.d. (twice per day) and t.i.d. (three times per day) dosing, making it tedious and burdensome for owners to constantly administer thyroid medication.

  • Methimazole: In terms of cost, methimazole is considered inexpensive and effective for the treatment of feline hyperthyroidism. Most cats require b.i.d. (twice per day) dosing schedules, meaning owners will be responsible for remembering to provide a morning and afternoon/evening dose.  Unfortunately, due to the fact that it is known to have a poor taste, most cats dislike taking it and/or are uncooperative.  In some cases, cats may even vomit after administration of their medication.

Medications also may have side effects that the cat dislikes: bone loss, diarrhea, fatigue, vomiting, etc.  Since you cannot know the subjective experience of your cat, it may be wise to consider that he/she may be experiencing unwanted side effects (or reactions) from the medication that you haven’t overtly observed.  Plus, these medications will require administration for the remainder of your cat’s life – which could be awhile.

Some recommend to use these medications in the format of transdermal gels.  While transdermal gels are considered feasible treatments, they are not devoid of risk nor side effects.  The rate of transdermal gel absorption may be subject to variation and trigger side effects.  Most veterinarians agree that there are usually favorable options to medications for the treatment of feline hyperthyroidism.

Dietary Iodine Restriction

In many cases, modification of dietary iodine can affect thyroid function of cats.  There is scientific evidence to suggest that short-term modification of dietary iodine significantly affects serum thyroid hormone concentrations.  It is understood that when a cat receives too much iodine, hyperthyroidism may occur; iodine is considered essential for the secretion of thyroid hormone.

If your cat is suffering from hyperthyroidism, a potentially therapeutic intervention to consider is the restriction of dietary iodine.  By restricting dietary iodine, the thyroid of your cat should produce less thyroid hormones.  Many pet owners have adopted the usage of “Hill’s Prescription Diet Y/D Feline – Thyroid Health.”

There is evidence to suggest that iodine restriction can normalize T4 levels in hyperthyroid cats.  This particular diet provides iodine levels at 0.32 mg/kg – considerably lower than the minimal daily requirement in adult felines at 0.46 mg/kg.  After a month of this particular diet, most cats will exhibit normalization of T4 concentrations in the serum.

Adherence for 2 to 3 months should restore normative T4 levels in nearly 100% of cats with moderate cases of hyperthyroidism.  It is unclear as to whether this dietary intervention would prove efficacious in cases of cats with severe hyperthyroidism.  That said, management of hyperthyroidism by restricting dietary iodine is far from utopian.

There are countless drawbacks associated with an iodine-restricted diet for the management of hyperthyroidism in cats.  These drawbacks should be considered prior to assuming that an iodine-restricted diet is optimal for your particular cat.

  • Not a cure: The restriction of dietary iodine is not a “cure” meaning you can’t simply restrict iodine for awhile, then stop, and expect your cat’s thyroid to remain normal. As soon as you lapse on the iodine restriction, your cat’s hyperthyroidism will return in full force.  In some cases, it may even return to a more severe extent than before.
  • Strict adherence: Restriction of dietary iodine requires strict adherence to be effective. If you feed your cat any treats, table scraps, or additional cat food – his/her hyperthyroidism is likely to return.  Adherence may be particularly difficult for owners of multiple cats in feeding will need to remain separated.
  • Long-term effects: The long-term effects of an iodine-restricted diet remain unknown. Some speculate that as a result of low iodine and high TSH, cats may be more susceptible to thyroid goiter, tumor growth, and various cancers.  Potential deleterious long-term complications should be considered and discussed prior to adoption of this diet.
  • Nutritional deficits (Unhealthy): An iodine-restricted diet requires that cats consume foods low in animal protein with cheap, unhealthy “fillers.” The lack of animal protein is detrimental to the functioning of the cat and may result in muscle wasting.  Additionally, many of the unhealthy “fillers” are goitrogenic compounds (soy, gluten, etc.) that may worsen underlying hyperthyroidism.  Cat owners should consider the nutritional deficiencies associated with this diet.
  • Side effects: Although restriction of iodine can help reduce hyperthyroidism, iodine plays an important role in numerous physiological processes and functions as an antioxidant. Cats with low levels of iodine may develop other health problems and functional abnormalities as a result of dietary restriction.

Understand that an iodine-restricted diet may be a better fit for some cats compared to others.  Adherence to the low-iodine diet is may be optimal among cats that wouldn’t benefit from treatment of thyroid tumors with radioiodine and/or find that their cat is unable to tolerate oral treatment and its associated side effects.

Partial or full thyroidectomy (Surgery)

Another option, if deemed suitable, is a surgical partial thyroidectomy.  Partial thyroidectomy involves extraction of a specific portion of thyroid glands in attempt to normalize thyroid function.  The major pitfall associated with using a partial thyroidectomy to treat hyperthyroidism is that the surgeon must be highly skilled and accurate with his removal or the cat may deal with deleterious lifelong consequences.

Removing too much of the thyroid gland will result in lifelong hypothyroidism (underactive thyroid), which in turn will require medication.  If an insufficient amount of the thyroid gland is removed, cats will remain stuck in a state of hyperthyroidism (albeit less severe than pre-surgery).  In the event that the cat has advanced thyroid cancer, a veterinarian may recommend a complete thyroidectomy (or removal of the entire thyroid).

It should also be noted that a partial or full thyroidectomy may affect parathyroid glands responsible for calcium regulation.  Should parathyroid glands could get injured during the procedure, this may cause dysregulation of calcium thereafter.  Furthermore, injuries to adjoining areas (around the thyroid) may affect nerve function, thereby affecting feline eyesight and/or vocalizations.

A thyroidectomy is generally reserved for cats in which thyroid cancer is the cause of overactive thyroid and requires anesthesia.  In some cases, multiple surgeries are required for normalization of thyroid function.  Approximately 1 of 10 cats die as a result of this surgery and/or surgery-related complications.  Due to the risks associated with a partial or full thyroidectomy, this should be considered a last-line option when all other interventions for hyperthyroidism are deemed unfeasible.

Monitoring Hyperthyroidism in Cats

If your cat has been diagnosed with hyperthyroidism and received any form of treatment, it is important to follow-up with regular veterinarian check-ups.  Within 2 to 3 weeks after initiation of treatment, a follow-up evaluation with additional bloodwork is usually advised.  Follow-up visits are necessary to determine whether a specific treatment (e.g. antithyroid medication, radioiodine therapy, etc.) has maintained efficacy in terms of normalizing thyroid concentrations.

Assuming your cat’s thyroid checks out as normal in the initial few follow-up appointments, your veterinarian may recommend a longer time gap between your next follow-up visit.  That said, the amount of time recommended between visits for blood testing may be contingent upon the general overall health of your cat and the specific intervention that was utilized for hyperthyroidism.  A cat taking medication and/or eating a low-iodine diet may require more regular bloodwork than a cat who responded well to radioactive iodine therapy.

That said, if all appears to be going well and your cat’s thyroid concentrations are regarded as normative, check-ups every 3 to 6 months may be recommended.   You’ll want to consult your veterinarian to determine how frequently your cat’s thyroid levels should be reassessed.  It is up to you as an owner to beware of potential signs of resurgent hyperthyroidism after treatment.

Risk factors for Hyperthyroidism in Cats

There are some factors that appear to increase propensity of cats to develop hyperthyroidism.  Researchers have discovered that things such as: cat breed, cat food, environmental toxins, litterbox usage, topical preparations, etc. – can all increase likelihood of certain cats to develop an overactive thyroid.  If you want to take steps to potentially reduce the likelihood that your cat will become hyperthyroid, you should understand risk factors.

  • Age: It’s no secret that older cats are at increased risk of hyperthyroidism compared to younger ones. If your cat is over the age of 10, risk of hyperthyroidism is innumerably greater than a newborn baby cat.  As cats age, their physiological function deteriorates and the thyroid has a tendency to become overactive.
  • Breed: Two breeds appear to be at considerably lower risk of developing hyperthyroidism than others, namely: Siamese and Himalayan cats. The Siamese and Himalayan breeds are considered genetically similar, and therefore may possess a particular gene or set of genes that protect against onset of hyperthyroidism.  Although Siamese and Himalayan cats are at reduced risk of hyperthyroidism, all mixed-breed cats appear to be at increased risk.
  • Canned cat food: The type of food you’re feeding your cat may be causing hyperthyroidism. Not only could the iodine content be too high/too low, but the food itself may contain toxins such as PBDEs (in the can and food) and BPA (in the lining of the cans – which could leech into the food).  Cats that are given commercially canned food are at 2-fold to 3-fold the risk of developing hyperthyroidism compared to those who aren’t.
  • Ectoparasite preparations: If you’re constantly slathering flea and tick removal on your cat, you may be increasing his/her risk for hyperthyroidism. There is evidence to suggest topical ectoparasite formulations may impair thyroid function.  Cats who aren’t exposed to these ectoparasite protectants don’t become hyperthyroid as often.
  • Litterbox usage: Research suggests that cats who use a litterbox develop hyperthyroidism at approximately 3-fold the rate of those who don’t. Perhaps the fact that cats who don’t use a litterbox are outdoors more frequently (breathing in fresh air) improves their overall health.  On the other hand, usage of a litterbox may expose them to toxins within the litter and their own fecal excretion.
  • Toxic exposures: There is ample evidence to suggest that your cat is at increased risk for hyperthyroidism if exposed to PBDEs and BPA. Understand that exposure to environmental toxins commonly occurs outdoors, but also may occur indoors such as via excessive dust, furniture, and canned food.  The greater the number of toxins to which your cat is exposed, the more likely he/she will experience health problems such as hyperthyroidism.
  • Source: http://www.ncbi.nlm.nih.gov/pubmed/19689668
  • Source: http://www.ncbi.nlm.nih.gov/pubmed/22352329
  • Source: http://www.ncbi.nlm.nih.gov/pubmed/10449223

Has your cat been diagnosed with hyperthyroidism?

If your cat has been diagnosed with hyperthyroidism, share a comment below.  Mention when you first started noticing symptoms and how long these symptoms progressed before the diagnosis of hyperthyroidism was made by a veterinarian.  To help others get a better understanding of your cat’s situation, share some details such as: his/her age, breed, and any other health conditions that he/she has.

Following the diagnosis of hyperthyroidism, what specific treatment did your veterinarian recommend?  Note whether you pursued the recommended treatment as whether it was effective.  Do you believe that your cat was genetically prone to hyperthyroidism or do you believe that certain factors such as: canned food, environmental toxins, litterbox usage, etc. – caused it?

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{ 4 comments… add one }
  • Kirsten November 28, 2016, 10:38 pm

    Very interesting article. I adopted my 14 year old tuxedo cat 6 months ago, he’s taking 1 tablet a day hiddden in a soft treat. He has shown some symptoms in the past, and I understood that a pill a day for life was what was needed to keep him healthy. After reading this and other articles, it would seem that the radiation therapy is highly beneficial to potentially reverse symptoms so that the cat can live a healthier longer life. I’m certainly going to look into this. Thank you.

  • PK August 20, 2016, 3:28 am

    Ezri: female; spayed; almost 16 y/o domestic long hair tabby. Strictly indoor all her life. Approximately two years ago, her almost 17 y/o “sister”, Kes, was diagnosed hyper-T after dramatic and sustained weight loss and ravenous appetite. We began noticing mild hyperT symptoms in Ezri approximately five months ago (increased drinking/urination, increased hunger, moderate weight loss). Mentioned to vet in April of 2016; dismissed and not tested.

    Two days ago Ezri began exhibiting neurological symptoms that indicated the possibility of Feline Hyperesthesia Syndrome. Vet did full blood panel and commenced her on phenobarbital. Normal T4 levels (for our lab) are 9-32; Ezri’s level 392. Conclusively hyperT. Treatment on methimazole commenced today; 2.5mg, b.d.).

    Now we wait and see if hyper-T treatment resolves neurological symptoms as well…

  • Julie July 24, 2016, 3:53 am

    11 year old female cat, drinking water continually, always hungry and weight loss. Labs done and T4 level was >10. Cat was started on Hills Thyroid Care y/d diet and is being fed it in both dry form and meat. Will recheck T4 levels in several weeks to see if any change has occurred. Cat is very loving and friendly, but is ill appearing due to weight loss.

  • Tammy June 5, 2016, 2:56 pm

    Noticed increased foamy and liquid vomiting a few years ago, but vet never diagnosed thyroid issues with our cat Lincoln. We noticed a lump in his neck and after 2 vets as we had a second opinion. Our new vet suggests surgery. We are unsure what to do because of risks with surgery the expense and success rate and life span after surgery. Even after research we don’t know what to do.

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