What's the Connection Between Thyroid and Weight Gain?

January is thyroid awareness month, and public interest in the thyroid is on the rise, especially as scientists have linked thyroid issues with infertility in women. The thyroid is a butterfly shaped gland that produces regulatory hormones for the endocrine system. It regulates BMR, or basal metabolic rate, which is what people refer to when they say they have a “high” or “low” metabolism. That said, the thyroid gland’s role in weight gain may be more complicated than expected.

The two main conditions surrounding irregularities in the thyroid gland are very similar-sounding: hypothyroidism and hyperthyroidism. However, regarding weight gain and other symptoms, they have many key differences to be aware of.




Hypothyroidism means having an underactive thyroid. It’s estimated that 10 million Americans and up to 10 percent of American women live with this condition, which slows down their body’s metabolism.

The relationship between thyroid and weight is much more extreme for those with overactive thyroids (hyperthyroidism) than those with underactive thyroids. According to the American Thyroid Association, an underactive thyroid usually only accounts for about 5-10 pounds of a patient’s weight gain. This extra weight is typically due to excess salt and water rather than stored fat.

Other relevant symptoms to look out for include constipation, depression, fatigue, muscle weakness, and pain/swelling of joints.

Treating Hypothyroidism

The goal of treating hypothyroidism is to get a patient’s levels back to the normal range. Doctors can prescribe T4 (levothyroxine sodium) which is a synthetic version of what your body already produces. Typically, doctors begin with a small dose and monitor patients closely to see how long a patient needs treatment. Patients should not expect to lose more than 10 percent of their body weight during treatment, since much of their excess weight is due to salt and water retention. Once treatment ends, a patient’s ability to lose or gain weight will return to that of a person without a thyroid condition.


Hyperthyroidism means having an overactive thyroid gland. This means the endocrine system produces too much of the thyroid hormone. These patients may require more calories than a typical person and have an increased appetite. The amount of excess calories they need is dependent on the severity of their diagnosis. If they do not meet the number of calories they need, they may experience extreme weight loss. On the other hand, it is possible that with an increased appetite, a hyperthyroidism patient may eat more than their body requires, and end up gaining weight.

More relevant symptoms include the inability to sleep, shaking, increased heart rate, increased frequency of bowel movements, and seizures.

Treating Hyperthyroidism

A wide variety of treatment options are available for hyperthyroidism, so patients should consult their doctor and decide which is best for them. These include drugs that act as antithyroid agents, radioactive iodine to destroy thyroid cells, or surgery to partially remove the thyroid gland. In addition to one of those methods, hyperthyroidism patients may also receive a prescription for beta-blockers to block the effect of the thyroid hormones.



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