Synthroid (Levothyroxine) is a medication commonly prescribed for the treatment of hypothyroidism, and is also a popular intervention for goiters, nodular thyroid disease, and thyroid cancer. Administration of synthroid reverses neurophysiological irregularities associated with insufficient thyroxine (T4) and reduces concentrations of TSH (thyroid-stimulating hormone). Within several months of synthroid administration, many individuals report improvements in mood, energy, and overall well-being.
Although synthroid is a synthetically-engineered form of thyroxine called “levothyroxine,” and isn’t bioidentical to endogenously secreted thyroxine (via properly functioning thyroid glands), it is similar enough to improve hypothyroidism biomarkers and symptoms. Assuming the dosage of synthroid is properly calibrated for the specific user, severe side effects and/or adverse reactions are unlikely. Most people are able to tolerate synthroid over a long-term (many years) without complaints of unwanted side effects.
Despite the favorable tolerability of synthroid for most users, a subset of users report significant unwanted synthroid side effects, one of which is often weight gain. It is unclear as to how synthroid may cause weight gain as most literature suggests that it is “weight neutral” or unlikely to affect body weight. However, there is some evidence to suggest that older adults (ages 45 and up) may gain weight as a result of synthroid treatment.
Synthroid (Levothyroxine) and Weight Gain
Firstly, it is important note that most medical literature suggests that synthroid does NOT cause significant weight gain. However, assuming you haven’t made any profound changes in eating habits, activity/exercise level, etc. while taking synthroid, yet you’ve packed on a noticeable amount of poundage – it’s probably safe to assume that it was triggered by the drug. Just because the majority of synthroid users are unlikely to gain weight, does not mean that synthroid-induced weight gain should be dismissed as a medical impossibility.
There’s never a one-size-fits-all response to medications – responses (and especially synthroid side effects) are subject to interindividual variation. While most people may not gain weight from synthroid, you may happen to be the outlier who ends up packing on the poundage; possibly to a significant extent. It may be frustrating to inquire about synthroid as the culprit for your weight gain to a medical professional, especially if you are told “synthroid does not cause weight gain.”
This may lead you assess any recent changes in: dietary intake, exercise level, sleep, stress, etc. – only to realize that nothing much has changed. At this point, you may want to talk to your doctor about going without synthroid and/or switching to another medication to see if your weight normalizes. That said, you should keep in mind that temporary fluctuations in body weight may be common over the first few weeks or months of synthroid treatment; these may subside with continued synthroid administration.
How Synthroid May Cause Weight Gain (Possibilities)
There are numerous possible ways in which synthroid administration could cause weight gain in a subset of users. Understand that there may be interindividual variation in the causative factors responsible for the weight gain. Some individuals may gain weight as a result of the fact that their appetite increases while taking synthroid, yet another person may realize that synthroid causes bloating and/or changes in gut bacteria that lead to weight gain.
- Appetite increase: Though some individuals find synthroid beneficial for reducing appetite, others notice a significant increase in appetite during synthroid treatment. Any increase in appetite, regardless of whether small or large, will increase the likelihood that you’ll consume more food (and calories throughout the day). The more food that you consume throughout the day, the more likely you’ll be to gain weight.
- Bloating: A side effect that synthroid users report is stomach bloating characterized by heightened build-up of fluid and/or gas. If you feel bloated while taking synthroid, you’re certainly not alone. Some people report such severe bloating, that they appear as if they’re pregnant. It may be necessary to consider that the extra weight retained as a result of bloating may be perceived as minor weight gain.
- Constipation: In addition to bloating, some individuals note that synthroid induces constipation. If you find it difficult to pass a bowel movement, some of the increased weight that you notice may be a result of your constipation. It may be helpful to intervene with increased dietary fiber, hydration, and/or laxative agents. Keep in mind that constipation is unlikely to cause a significant amount of weight gain, but may contribute slightly to your overall weight increase if you experience it.
- Food cravings: Although fairly rare, some users report hunger pains and/or substantial food cravings while taking synthroid. You may find yourself craving unhealthy foods such as: candies, cookies, and various sugary treats. Assuming you are unable to resist these cravings, you’re probably going to gain some weight. To minimize your chances of falling victim to food cravings, you may want to avoid purchasing any foods that reinforce these cravings during synthroid treatment.
- Gut bacteria: Pharmaceutical medications, especially synthetic ones like synthroid, may be altering your gut microbiome. In other words, regular administration of synthetic thyroxine (levothyroxine) may alter the densities of particular gut bacterium. As a result of changes in gut bacteria associated with synthroid, users may gain weight without any dietary or exercise changes solely resulting from bacteria in the gut.
- Low energy: Not everyone feels energetic and spry after taking synthroid. Some individuals report feeling lower energy after taking synthroid than prior to treatment. If you notice that you become increasingly fatigued and/or lethargic throughout treatment, chances are that you’ll gain some weight. Fatigue makes it difficult to remain physically active, engage in regular exercise (to burn calories and keep metabolism high), and also difficult to make healthy decisions regarding foods to eat.
- Neurotransmission: Regular administration of synthroid may decrease concentrations of dopamine. Precursors such as tyrosine appear to decrease in the brain of rodents regularly administered synthroid. It is plausible to consider that humans who’ve taken synthroid for a long-term may end up with low dopamine (at least in certain regions of the brain). Reductions in dopamine could lead to poorer appetite control, feeling less satiated after eating, etc. – possibly contributing to weight gain.
- Sleep disturbances: It is understood that a quality night’s sleep is beneficial for maintaining a healthy body weight. Adequate deep sleep keeps the metabolism within a healthy range and allows the body to produce hormones to prevent weight gain. Lack of sleep increases ghrelin (the hunger hormone) and may disrupt other metabolic processes, ultimately leading to weight gain. Since synthroid administration may cause insomnia and sleep disturbances in some users, its interference with a good night’s sleep may indirectly contribute to weight gain. (Source: http://www.ncbi.nlm.nih.gov/pubmed/18564298).
- Synthetic reaction: The fact that synthroid is a synthetic, rather than bioidentical hormone is problematic for many users. The chemical structure of synthroid differs from endogenously produced thyroxine (T4), and as a result, it may provoke abnormal and unpredictable neurophysiological reactions in users. Weight gain may be one of many possible implications associated with using a synthetically-manufactured hormone, as opposed to one that is endogenously secreted (or even bioidentical).
Variables that may influence weight gain from Synthroid (Levothyroxine)
Whenever contemplating the weight you’ve gained from synthroid, it may be helpful to consider influential variables. Examples of such variables to consider that may affect weight gain include: synthroid dosage, duration of administration, other medications or drugs, and your lifestyle. Some users may find that they were gaining weight from an improper synthroid dosage, while others may realize that weight gain could be caused by other pharmaceutical medications.
The dosage of synthroid that you’re taking may affect whether you’re likely to gain weight. Individuals taking extremely high dosages are more likely to lose weight than gain weight. High dosages of synthroid are thought to increase basal metabolic rate (BMR) and the number of calories burned at rest.
On the other hand, those taking low dosages of synthroid may end up gaining weight as a result of inadequately treated hypothyroidism. At extremely low dosages, you may not attain enough synthroid to counteract the metabolic slowing that occurs with hypothyroidism. It may take your doctor awhile to find the optimal dosage to get your TSH levels within a normative range.
Synthroid dosing is known to have an NTI (narrow therapeutic index). In some cases, even high-dose users may end up gaining some weight – assuming their neurophysiology reacts unusually to the synthroid. A properly calibrated dosage by a medical professional (e.g. endocrinologist) should help reduce likelihood of weight gain (or loss) while taking synthroid.
Duration of administration
The duration over which you’ve taken synthroid (levothyroxine) may affect how likely you are to gain weight. It is possible that some users may gain weight over a short-term (e.g. less than 3 months of treatment), while others may notice some weight gain over a longer-term (e.g. 3+ months or years). The shorter the duration over which you’ve taken synthroid, the more likely you are to experience transient weight fluctuations.
Short-term: Those that have taken synthroid for just a short-term may be undergoing a transient physiological adjustment in which some weight is gained. However, with continued treatment (e.g. over the course of months and/or years), body weight is likely to normalize and weight that was initially gained may be lost. If you’ve gained weight over the short-term (e.g. less than 3 months), don’t rule out the fact that you may eventually lose this weight.
Long-term: It is possible to consider that individuals who didn’t gain weight over a short-term, may end up gaining some weight over the long-term. If you lost weight over the short-term with synthroid, it could’ve been a result of a neurophysiological overreaction. Additionally, short-term weight loss may be a result of too high of a synthroid dosage. Once the neurophysiology adjusts to synthroid (over a long-term) and/or an abnormally high dose is reduced, users may gain back some of the excess weight that was initially lost.
When taking synthroid, it is necessary to consider that other medications and/or dietary supplements may be affecting your weight. Certain medications and/or supplements may decrease the absorption of synthroid (especially if you take them within 4 hours of synthroid). This may lead you to gain weight while taking synthroid due to the fact that you’ve reverted back to a state of (subclinical) hypothyroidism as a result of poorer synthroid absorption.
It is also necessary to reflect upon the fact that other medications may be the sole cause for your weight gain. Although you may suspect that your weight gain is caused by the synthroid, other medications and/or dietary supplements should be ruled out as potential causes. If you have any questions as to whether another drug (or supplement) is leading to weight gain – talk to a medical professional.
It is necessary to consider that individual factor such as: dietary intake, exercise/activity level, sleep (quality/quantity), and stress – can each affect body weight. A person’s age and genetics may also affect how they respond to the synthroid and whether they gain weight. If you’ve gained weight while taking synthroid, you may need to assess whether: diet, exercise, sleep, and/or stress changed throughout your treatment.
- Age: Research indicates that adults over 45 year of age are most likely to gain weight with synthroid treatment. Studies suggest that the average amount of weight gained among older adults is 4.85 lbs (2.2 kg). While this isn’t necessarily a large amount of weight gain, it may be noticeable in certain users. If you are a younger individual (under the age of 45), significant weight change is considered unlikely.
- Dietary intake: If your diet changed significantly after starting synthroid and you’ve gained weight, it is impossible to chalk your weight gain up to the synthroid. On the other hand, if your diet has stayed nearly the exact same during treatment (as it was pretreatment), diet can be ruled out as an influential factor. That said, you should make a conscientious effort to eat healthy foods to minimize likelihood of weight gain.
- Exercise: Some individuals end up gaining weight while taking synthroid because they stop exercising and/or become physically less active. If you stop working out at some point during synthroid treatment and gain weight, it’s most likely not a result of the synthroid. Getting plenty of exercise and staying physically active during synthroid treatment is often helpful for weight management.
- Genetics: It is possible that individual genetics may predict responses to synthroid, as well as weight changes. Certain individuals experience severe side effects while taking synthroid, yet others don’t experience any side effects – this may be a result of genetics. Your genes may determine how you react to synthroid in terms of weight change.
- Sleep: Although synthroid can cause sleep disturbances (which could indirectly lead to weight gain), poor sleep hygiene may also cause weight gain. If you don’t get enough sleep as a result of your personal decision to stay up (e.g. to hang out with friends, use the computer, etc.) – realize that you may gain weight. Managing sleep hygiene is important to ensure that you won’t gain weight while taking synthroid.
- Stress: Your stress level experienced while taking synthroid could affect whether you end up gaining weight. Those who experience a sudden increase in stress may find it difficult to get proper sleep and/or may end up gaining weight as a result of stress-induced hormonal changes. You may want to avoid blaming synthroid for weight gain if you’ve recently experienced high stress.
Synthroid (Levothyroxine) & Weight Gain (The Research)
There is limited scientific evidence to support weight gain associated with synthroid. Few studies have taken the time to investigate whether synthroid (levothyroxine) administration could cause weight changes, especially when administered over a long-term. There’s only modest evidence from a 2010 study suggesting that older adults (ages 45+) may gain a little bit of weight from synthroid treatment.
2010: The effects of levothyroxine substitution on body composition and body mass after total thyroidectomy for benign nodular goiter.
Research by Ozdemir et al. (2010) documented the effects of levothyroxine (synthroid) on body mass index and composition among individuals subject to a total thyroidectomy (removal of the thyroid gland via surgery). Their motive for investigating how thyroxine effects body mass and composition stemmed from the fact that many thyroidectomy patients complain of weight changes following surgery.
Researchers measured BMI and composition of all patients prior to the thyroidectomy, and then again 3-weeks post-surgery. After the thyroidectomy, all patients received levothyroxine at dosages of 50 to 200 mcg per day. All dosages were adjusted with the intent of optimizing TSH, FT4, and FT3 levels.
Upon comparison of the pre-thyroidectomy measures in body weight, there appeared to be no significant change in weight among the 22 participants that completed the study. However, upon closer analysis, older adults (ages 45-64) experienced slight weight gain as a result of levothyroxine. Older adults gained an average of 4.85 lbs (2.2 kg), whereas younger adults (under 45 years of age) didn’t experience any weight changes.
Although this is a small-scale study and it is unclear as to whether results can be generalized to other older adults taking levothyroxine (or non-thyroidectomy patients), and/or whether weight loss would be more significant over a longer duration – the study provides some evidence to suggest that levothyroxine may cause weight gain. Follow-up research is necessary to confirm preliminary findings in this study. That said, older adults (particularly thyroidectomy patients), may gain weight during levothyroxine treatment within 3-weeks of starting treatment.
- Source: http://www.ncbi.nlm.nih.gov/pubmed/21077724
How much weight will you gain from synthroid (levothyroxine)?
Assuming you gain weight from taking synthroid (levothyroxine), it is unlikely to be an overwhelming amount. Even if you do gain quite a bit of weight when starting treatment, keep in mind that it may take your neurophysiology awhile to accommodate and/or adapt to the drug. Once your body adapts to the presence of synthroid, you may end up losing some of the weight that you initially gained.
However, others may notice that they actually lose weight when starting treatment, but over a long-term, they end up gaining a significant amount of weight. Although it’s difficult to prove that the synthroid was the cause of weight gain over a long-term, it should be considered as a possibility. User reports have documented weight increases throughout treatment ranging from 5 lbs to 30 lbs, however, it is unclear as to whether other factors played a role in the weight gain.
Does everyone gain weight from taking synthroid (levothyroxine)?
No. In fact, many people report weight loss from synthroid treatment. Individuals with severe hypothyroidism often gain weight due to slowing of BMR (associated with hypothyroidism). When basal metabolic rate is sped up to a normal range after treatment with synthroid, most users end up losing some weight.
Additionally, many individuals taking synthroid notice zero change in weight or remain “weight neutral” throughout treatment. Don’t be surprised if your weight stays relatively similar throughout treatment as it was pre-treatment. Only a small subset of synthroid users are thought to gain weight, and in these cases, it is unclear as to whether the weight gain was caused directly by the medication.
Have you experienced weight gain from synthroid (levothyroxine)?
If you’ve taken synthroid, leave a comment mentioning whether you’ve gained weight as a result of treatment. To help others get a better understanding of your situation, mention: the amount of weight you gained, your synthroid dosage, as well as how long you’ve been taking it. Do you believe that the weight you’ve gained is a solely from the synthroid or from a combination of factors (including synthroid)?
For those that experienced short-term weight gain (e.g. gain in less than 3 months), did it stabilize with long-term synthroid treatment? Is there any way you can be sure that the weight gain was from synthroid? Did you rule out other potential causative factors such as changes in: sleep, diet, other medications, exercise, etc. – to know that the weight gain was from synthroid?
If you have any particular theories as to how synthroid may cause weight gain, include an explanation in your comment. Additionally, if you have any tips for regulating body weight while taking synthroid that you’ve found effective, be sure to mention them. Although most users will remain weight neutral while taking synthroid, you may be one of the (unlucky) few who ends up gaining weight.