Synthroid (Levothyroxine) is among the most popular pharmaceutical medications in the United States; over 120 million prescriptions were filled for the drug within the past year. It is a synthetically engineered form of thyroxine (T4), an endogenous thyroid hormone produced by follicular cells within the thyroid gland. Among individuals diagnosed with hypothyroidism, the body is unable to produce adequate T4 (thyroxine) via the thyroid gland, which has deleterious effects upon DNA transcription, protein synthesis, and a multitude of neurophysiological processes.
As a result, individuals with hypothyroidism often exhibit symptoms such as: cognitive deficits, depression, dry skin, fatigue, malaise, and weight gain. Regular administration of synthroid (levothyroxine) is thought to reverse the aforementioned symptoms of hypothyroidism and improve functionality of the user. After several months of properly-dosed synthroid, users often report: clearer thinking, increased energy, mood improvements, and perhaps most noticeably – weight loss.
Weight loss is often among the most discussed synthroid side effects due to the fact that nearly everyone wants to look good and maintain a healthy physique. Additionally, many individuals had gained a significant amount of weight prior to using synthroid (as a result of hypothyroidism), a condition which slows metabolism. When metabolic processes are corrected as a result of synthroid, users end up losing weight and the weight loss comes as a pleasant surprise.
Synthroid (Levothyroxine) and Weight Loss: A Common Experience
If you’ve lived with hypothyroidism, there’s a chance that you may have gained some weight as a result of the condition. The chief purpose of thyroid hormone is to regulate the body’s metabolism, and when an individual is struggling with a thyroid deficiency, basal metabolic rate (BMR) plummets. Individuals with hypothyroidism may: cut calories, eat a healthy diet, exercise plenty – yet still end up gaining weight due to their slower basal metabolic rate; fewer calories are being burned at rest.
During treatment with synthroid, the body receives a synthetic version of thyroxine (T4) which upregulates the hypothyroid-induced metabolic downregulation and facilitates weight loss. Users that are prescribed an optimal dosage of synthroid should lose all weight that was gained specifically as a result of their hypothyroidism. Weight loss from synthroid is usually most prominent during early stages of synthroid treatment because the body hasn’t yet adapted to the increase in basal metabolic rate (BMR).
As synthroid treatment is continued for an extended duration, the body will have adjusted to the increase in basal metabolic rate (BMR), and additional weight loss will subside. For some users, weight loss may subside after a few months of treatment, yet for others it may take a year for the weight loss to diminish. The amount of weight loss that occurs from synthroid, as well as how quickly it occurs, will likely be contingent upon how long you lived with untreated hypothyroidism and the severity of your hypothyroidism.
How Synthroid (Levothyroxine) May Cause Weight Loss
There are many possible ways in which synthroid may contribute to weight loss. In most cases, weight loss resulting from synthroid usage should be chalked up to the fact that it corrects an abnormal and/or dysfunctional basal metabolic rate (BMR) associated with hypothyroidism. It is the correction of BMR (by promoting an increase) that leads to weight loss. Other ways in which synthroid may cause weight loss include: appetite reduction, cognitive enhancement, increased energy, altered neurotransmission, and side effects (e.g. diarrhea).
- Appetite reduction: Many synthroid users report that the drug decreases and/or suppresses their appetite. Assuming you feel less hungry than usual while taking synthroid, there’s a good chance that you’ll probably eat less food and end up losing some weight. In a subset of individuals diagnosed with hypothyroidism, an overwhelming appetite is problematic and leads to constant overeating. Synthroid administration may reduce a large appetite and decrease the urge to eat.
- Cognitive function: It is understood that those with hypothyroidism often exhibit brain fog, inability to think clearly, and poor cognitive function. As a result, many people with hypothyroidism make impulsive food choices and lack a nutrient-dense diet. Poor nutrition exacerbates existing cognitive deficits and perpetuates future suboptimal dietary choices. Upon correction of hypothyroidism-associated cognitive deficits (as a result of synthroid treatment), individuals are able to make better decisions about the foods that they consume and exert more overall self-restraint to avoid unhealthy food choices.
- Decreased food cravings: In some individuals, hypothyroidism may contribute to food cravings, especially for junk food (e.g. artificial sweeteners, sugars, simple carbs, etc.). With regular administration of synthroid, some individuals may find that food cravings subside. You may feel as if you no longer are constantly craving carbs, cookies, candies, etc. – all throughout the day. Assuming you have fewer overall food cravings, you’re likely to consume less unhealthy “junk food” and could end up losing some weight.
- Energy increase: There’s a stark contrast in the energy level of an individual with an optimally functioning thyroid and someone who’s suffering from hypothyroidism. Most individuals with hypothyroidism slog through the day with considerable fatigue and grogginess, making it challenging and less appealing to stay active and/or exercise. When taking synthroid, your energy level may skyrocket (compared to what it was) and you may end up going to the gym, moving around more, etc. – all of which can burn calories, enhance your metabolism, and contribute to weight loss.
- Ghrelin reduction: A hormone responsible for regulating hunger, known as “ghrelin,” is abnormally high among individuals with hypothyroidism. Ongoing administration of levothyroxine (synthroid) appears to significantly reduce ghrelin concentrations. It is possible that the reduction of ghrelin as a result of synthroid administration leads users to feel more satiated with fewer food cravings than usual. (Source: http://www.ncbi.nlm.nih.gov/pubmed/23339003)
- Gut bacteria: There is some evidence to suggest that some cases of hypothyroidism may be caused by small-intestine bacterial overgrowth (SIBO) within the gut. It is known that a person’s gut bacteria can be predictive of whether they’re likely to lose or gain weight from eating a certain set of foods. One person with a specific gut microbiome may differ significantly in terms of weight loss/gain from a particular food – than a person with a different gut microbiome. It may be necessary to consider that administration of synthroid may alter a person’s gut microbe in such a way as to promote weight loss.
- Metabolism increase: The primary reason synthroid users experience weight loss is related to an increase in basal metabolic rate (BMR). As was already discussed, individuals with hypothyroidism tend to exhibit abnormally slow basal metabolic rates. When synthroid is taken, basal metabolic rate is corrected (or normalized) and weight loss occurs without much effort. This may result in weight loss without any change in lifestyle, dietary intake, etc. When you’re burning more energy at rest as a result of an increase in basal metabolic rate (compared to pre-synthroid treatment), weight loss is likely to occur.
- Neurochemistry: There is evidence to suggest that synthroid (levothyroxine) affects neurotransmission and neural activation (of particular regions). For this reason, it should be hypothesized that altered concentrations of neurotransmitters (e.g. monoamines) and/or changes in neural activation following synthroid administration – may contribute (directly or indirectly) to weight loss. It is understood that neurochemical changes can affect propensity to lose weight.
- Placebo effect: It may be necessary to consider that in a subset of synthroid users, the placebo effect contributes to weight loss. Some individuals may expect to lose weight while taking synthroid, and the weight loss becomes a self-fulfilling prophesy. Since the scientific literature reports that synthroid isn’t associated with significant changes in body weight (on average), yet many users still report weight loss – the placebo effect should be considered a potential contributing factor.
- Side effects: In some cases, users of synthroid may experience weight loss as a result of other side effects. For example, if synthroid is provoking round-the-clock diarrhea, the laxative effect will likely contribute to some degree of weight loss. Additionally, it could be that synthroid causes nausea and/or stomach aches – each of which may interfere with a person’s appetite, resulting in reduced food consumption. Consider that some of the weight loss may stem from other side effects, especially if they are severe.
Note: Although weight loss from synthroid is thought to primarily stem from increases in BMR, other potential causative factor should not be dismissed. Furthermore, the exact causes of synthroid-induced weight loss may be subject to interindividual variability. One individual may lose weight while taking synthroid primarily from BMR increases and diarrhea, while another may lose weight due to the fact that synthroid increased his/her energy enough to maintain a daily fitness routine.
Variables that may influence weight loss from Synthroid (Levothyroxine)
There are variables that may influence a bulk of synthroid-induced weight loss that you experience. These variables include things like: synthroid dosage, duration of administration, co-ingested medications, and other individual factors (e.g. stress level, sleep quality, etc.). When contemplating how much weight you’re likely to lose while taking synthroid, it may be helpful to consider these variables.
The amount of synthroid that you ingest on a daily basis can affect how much weight you’re likely to lose. The dosage is determined based upon the severity of an individual’s hypothyroidism, as well as his/her body mass index. In any regard, those taking higher doses of synthroid are likely to lose more weight than those taking extremely low doses.
This is due to the fact that high-dose users are likely to have severe hypothyroidism. Severe hypothyroidism may have resulted in significant weight gain prior to synthroid treatment. As a result, when the individual pursues synthroid treatment, he/she has a lot more weight that is easily lost upon treatment with levothyroxine (synthetic T4).
Additionally, the higher the dosage administered (based on a person’s body size), the greater the neurophysiological effect resulting from synthroid. When administered at too high of a dose, users may exhibit symptoms akin to hyperthyroidism, abnormally high BMRs (basal metabolic rates), and weight loss. The greater the dosage you’re taking, the more likely you are to end up losing some weight.
Those taking low doses of synthroid such as for subclinical hypothyroidism are less likely to lose a significant amount of weight. This is due to the fact that subclinical hypothyroidism is unlikely to cause significant weight gain in the first place. Since lower doses exert a less substantial neurophysiological effect, BMR may only increase to a modest extent – leading to a small amount of weight loss.
The duration over which you’ve been taking synthroid can affect how much weight you lose. Most individuals with severe hypothyroidism report noticeable weight loss after several months of treatment (e.g. short-term). In some cases, this weight loss may continue over a longer duration as dosage is reassessed and/or readjusted. In other cases, weight loss may be limited to a short-term and may gradually diminish and/or cease with a longer-term of synthroid treatment.
Short-term: Taking synthroid over an extremely short-term (e.g. a couple days) may not be long enough for noticeable weight loss. However, most individuals end up noticing some degree of weight loss after several weeks of synthroid treatment. Short-term weight loss is common due to the fact that your body is adjusting to a faster BMR as induced by the synthetic thyroxine (T4).
Long-term: Over a long-term of treatment, weight loss from synthroid is likely to subside. You may end up losing weight gradually over a period of months, but eventually your body will adjust to the regularly administered, daily dosage of synthroid. Following neurophysiological adjustment to synthroid, weight loss tends to diminish and body weight is often maintained. Additional weight loss after a long-term of treatment may occur among those who are subject to further increases in dosing.
To understand how much weight you’re likely lose while taking synthroid, it may be necessary to reflect upon the details of your hypothyroidism such as: how long you’ve lived with it and its severity. If you’ve been living with undiagnosed and/or untreated hypothyroidism for an extended duration, you may have gained a significant amount of weight as a result. This means you’ll stand to lose more weight during treatment with synthroid.
It is also important to consider that if your hypothyroidism is only of mild severity, it may not have significantly altered your weight in the first place. Since mild or subclinical hypothyroidism is often treated with low doses of synthroid, weight loss from treatment may be unnoticeable and/or insignificant. Those with severe hypothyroidism taking high doses of synthroid have the greatest chance of losing weight.
It may be necessary to consider that co-administered medications and/or supplements may affect how much weight you lose while taking synthroid. Some medications are known to interfere with the absorption of synthroid, leading to suboptimal therapeutic outcomes. Most medical professionals instruct patients to avoid certain drugs and/or supplements (e.g. multivitamins) while taking synthroid.
Co-administered substances may potentiate and/or inhibit weight loss associated with synthroid. For example, someone prescribed a pharmaceutical psychostimulant (e.g. Adderall) may lose weight from both the synthroid and the psychostimulant. It may also be necessary to consider that another medication and/or supplement that you’re taking is responsible for nearly all of the weight loss that you’ve experienced on synthroid.
Many people automatically assume that synthroid caused weight loss, without considering that it may have been from an entirely different pharmaceutical. That said, if you don’t experience much weight loss from taking synthroid, it is possible that another medication and/or supplement is causing weight gain, thereby interfering with synthroid-induced weight loss. Consider that synthroid and another substance may be effectively “canceling each other out” – resulting in weight neutrality or no change in weight.
Other individual factors
Everyone is quick to automatically assume that synthroid is the sole influencer of weight loss without considering critical individual factors. Individual factors that should always be considered as affecting synthroid-induced weight loss include: dietary intake, exercise, genetics, sleep, and stress level. Someone who starts taking synthroid, but also begins eating healthier, exercising frequently, getting better sleep, and lowering his/her stress – may experience weight loss from lifestyle changes.
These lifestyle changes may compliment the weight loss induced by synthroid, resulting in a more significant change in physique. On the other hand, someone who uses synthroid and: eats junk food, doesn’t exercise, fails to manage stress, and is sleep-deprived – may not lose any weight. Why? Because poor lifestyle habits such as sleep-deprivation are known to cause weight gain.
When taking synthroid you may want to consider how your lifestyle may have played a role in the weight loss. It should also be noted that genes (and expression) may make you more or less likely to lose weight from a medication like synthroid. Individuals with a particular gene (or lack thereof) may not lose any weight during treatment, while others with certain genes may lose a significant amount in response to the medication.
Synthroid (Levothyroxine) & Weight Loss: The Research
Although many individuals report losing weight while taking synthroid (levothyroxine), the scientific literature suggests that weight loss is unlikely. That said, there are only a few published studies investigating the effects of synthetic thyroxine (T4) on body weight. Additionally, some of these studies were conducted in horses rather than humans, making it difficult to draw any relevant conclusions.
2014: Changes in body weight after treatment of primary hypothyroidism with levothyroxine.
A study conducted by Lee et al. (2014) investigated the effect of levothyroxine (LT4) therapy on body weight of hypothyroid patients. Researchers specifically sought to determine the significance of weight changes after LT4 treatments were initiated for primary hypothyroidism. They set-up a retrospective cohort study that took place from January 2003 to February 2011 consisting of adults diagnosed with primary hypothyroidism.
A total of 101 participants completed the study, each of which had TSH levels exceeding 10 mIU/L. Prior to treatment with levothyroxine, researchers recorded participants’ weight and TSH concentrations. Approximately 24 months after the initiation of levothyroxine treatment, body weight and TSH levels were reassessed and compared to (pre-treatment) baseline recordings.
Results indicated that baseline average weight was around 175 lbs (79.6 kg) and after 5 months of levothyroxine treatment, average weight change was 0.22 lbs (-0.1 kg). Of the 101 participants, 52% lost weight – average weight loss was 8.4 lbs (3.8 kg). Researchers concluded that no significant changes in weight occur after initiation of levothyroxine (LT4) treatment.
- Source: http://www.ncbi.nlm.nih.gov/pubmed/24936556
2010: The effects of levothyroxine substitution on body composition and body mass after total thyroidectomy for benign nodular goiter.
A study conducted by Ozdemir et al. (2010) investigated the effect of thyroxine on body mass index (BMI) and body composition among patients who underwent a total thyroidectomy. Researchers noted that many thyroidectomy patients report weight changes and/or changes in bodily composition after the procedure. A total of 33 patients were recruited for the study, but only 22 individuals completed it (5 males, 17 females, ages 26 to 64 years).
Following the thyroidectomy, levothyroxine was administered at 50-200 mcg based on the particular patient. Dosages were adjusted based on postoperative plasma TSH, FT4, and FT3 levels – 3 weeks after surgery. After treatment with thyroxine, researchers recorded changes in: body weight, body mass index (BMI), and other physical measures.
Results indicated that levothyroxine administration had no effect on body composition. However, it appeared as though body weight was significantly altered by levothyroxine among the 14 participants over the age of 45 (ages 46 to 64). In the older adults, levothyroxine administration increased boy weight by an average of 4.85 lbs (2.2 kg).
Younger individuals (under the age of 45) didn’t appear to experience significant weight changes. Evidence from this study suggests that levothyroxine may cause weight gain in older adults (rather than weight loss). Since the study was small-scale with limited numbers of participants, it is unclear as to whether the results of the study accurately reflect weight changes among older adults taking levothyroxine; further follow-up research is warranted.
- Source: http://www.ncbi.nlm.nih.gov/pubmed/21077724
2008: Effects of long-term oral administration of levothyroxine sodium on serum thyroid hormone concentrations, clinicopathologic variables, and echocardiographic measurements in healthy adult horses.
A study conducted by Frank et al. (2008) sought to determine the effects of long-term oral levothyroxine (LT4) administration in adult horses with normative thyroid function. For the study, researchers utilized a total of 6 healthy adult mares. All horses were administered 48 mg per day of levothyroxine for a duration of 48 weeks.
Physical examinations of body weight were performed in 4 week intervals throughout the entire 48-week term. Throughout the 48-week term of treatment, horse body weight significantly decreased when compared to pre-treatment baseline. Results indicated that after 48 weeks of treatment, average horse body weight had decreased by 55 lbs (25 kg).
Additionally, researchers noted that by the end of the 48-week term, insulin sensitivity had significantly increased by approximately 1.9-fold. The greater the extent to which insulin sensitivity increased, the more significant the amount of weight loss that occurred. Researchers concluded that levothyroxine significantly reduces weight in healthy adult mares and may be helpful for the treatment of obesity and/or insulin resistance.
It should be noted that results from an animal (horse) study cannot and should not be generalized to humans. That said, it is possible to consider that humans who lose weight while taking levothyroxine may experience increases in insulin sensitivity. Since this study was conducted in healthy (euthyroid) horses, it is unclear as to whether similar effects could be observed among horses with hypothyroidism.
- Source: http://www.ncbi.nlm.nih.gov/pubmed/18167090
How much weight will you lose while taking synthroid (levothyroxine)?
There’s no set amount of weight that you can expect to lose while taking synthroid. Published studies in humans suggest that weight loss associated with synthroid treatment is unlikely. As of now, there is no significant evidence to suggest that you’re likely to lose weight from regular synthroid treatment over a period of 5 months.
There is some evidence to suggest that a subset of synthroid users may actually gain weight, particularly those who are “older adults” (ages 45+). However, there are numerous variables that should be considered whenever contemplating how much weight you’re likely to lose (or gain) while taking synthroid. Synthroid should be capable of reversing any weight you had gained as a result of hypothyroidism-induced metabolic changes.
If you believe that you gained around 20 lbs from untreated hypothyroidism, synthroid treatment should help you lose the 20 lbs. That said, synthroid will not reverse weight gained from things such as: eating unhealthy, excess calorie consumption, lack of exercise, high stress, poor sleep, and/or taking another medication. If you’re unable to lose weight from synthroid, it could be that your dosage is too low, you haven’t taken it for long enough, and/or the weight loss is being offset by an aforestated variable.
Does everybody lose weight while taking synthroid (levothyroxine)?
No. Clearly not everyone ends up losing weight from taking synthroid. While weight loss is more commonly reported than weight gain, it is not a guaranteed side effect. A subset of synthroid users remain relatively “weight neutral” or don’t fluctuate in body weight to a significant extent as a result of the medication.
Another percentage of users may actually gain weight as a result of synthroid. Whether you lose weight while taking synthroid may be contingent upon variables such as: your dosage, duration of treatment, and/or other medications ingested. Someone taking synthroid along with a psychostimulant may end up losing weight from the synergistic combination (each of which increase BMR). Other users taking antipsychotics may gain weight because the antipsychotic has a propensity to decrease BMR.
If you’re taking synthroid and/or about to start treatment, don’t expect any weight loss. Simply expect the medication to correct many of the neurophysiological abnormalities associated with hypothyroidism. The goal of synthroid treatment is to improve compromised functionality and/or well-being associated with hypothyroidism – not to lose weight.
Have you lost weight while taking Synthroid (levothyroxine)?
If you’ve taken synthroid, leave a comment mentioning whether you lost weight as a side effect. To help others get a better understanding of your situation, share some details such as: how much weight you lost while taking synthroid and how long it took to lose the weight. Discuss whether you believe that synthroid directly caused the weight loss OR whether synthroid indirectly helped the weight loss such as by giving you more energy to work out and/or make better decisions about foods that you eat.
Did you perceive the weight loss as being a favorable aspect of treatment? Or were you already relatively skinny/small-statured and perceived the weight loss from synthroid as being unwanted? Understand that if you’re a new synthroid user and haven’t been taking it for a long-term, you may lose weight over a period of months, but the weight loss may decline and/or subside thereafter.
This is because after awhile, your body adapts to the increase in BMR and neurophysiological stimulation provided by the synthroid. Shorter-term users are more likely to report major weight changes, but it is unclear as to whether these weight fluctuations can be maintained. To better understand individuals that are likely to lose weight from synthroid (levothyroxine), there is a need for additional scientific research.