Do thyroid medications cause weight gain?

From the Guidelines

Thyroid medications do not typically cause weight gain, and in fact, they often help with weight loss in people with hypothyroidism. According to a study published in the British Journal of Pharmacology 1, thyroid hormone replacement therapy can induce weight loss. The study suggests that thyroid hormone can slightly improve obstructive sleep apnea (OSA), which is often associated with weight gain.

When considering the effects of thyroid medications on weight, it's essential to distinguish between hypothyroidism and hyperthyroidism. In people with hypothyroidism, thyroid medication helps restore normal metabolism, leading to weight stabilization or weight loss as the body returns to proper function. However, if the thyroid medication dose is too high, causing hyperthyroidism, it might lead to increased appetite and potential weight gain.

Some key points to consider:

• Thyroid hormone replacement therapy can induce weight loss 1
• Thyroid medications help restore normal metabolism in people with hypothyroidism
• Incorrect dosing of thyroid medication can lead to weight changes
• Normalization of thyroid levels with medication can stabilize metabolism, which might feel like weight gain compared to a previous hyperthyroid state

It's crucial to monitor thyroid levels and adjust medication doses as needed to avoid weight changes. If you're taking thyroid medication and experiencing unexpected weight gain, it's essential to have your thyroid levels checked to ensure your dose is optimized.

From the FDA Drug Label

WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS • Thyroid hormones, including levothyroxine sodium, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. • In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. The answer to whether thyroid meds cause weight gain is not directly addressed. However, it is stated that in euthyroid patients, doses of thyroid hormones within the range of daily hormonal requirements are ineffective for weight reduction. This implies that thyroid meds do not typically cause weight loss, but it does not directly answer whether they cause weight gain 2.

From the Research

Thyroid Medications and Weight Gain

• The relationship between thyroid medications and weight gain is complex and has been studied in various research papers 3, 4, 5, 6, 7.
• A study published in 2021 found that patients with hypothyroidism who were treated with levothyroxine replacement therapy experienced symptom relief and normalized thyroid-stimulating hormone levels, but the study did not specifically address weight gain as a side effect of the medication 3.
• Another study published in 2001 found that patients who were treated for hyperthyroidism experienced significant weight gain, with a mean weight gain of 5.42 kg over a period of 24 months 4.
• A 2019 study found that normalization of thyroid function led to a decrease in weight in patients with hypothyroidism and an increase in weight in patients with hyperthyroidism, but the study noted that most patients remained in the overweight or obese range despite normalization of thyroid function 5.
• A 2021 study published in the European Thyroid Journal found that treatment of hyperthyroidism with antithyroid drugs was associated with a significant increase in body weight, with a mean weight gain of 5.2 kg over a period of 1 year 6.
• A 2025 study found that patients with post-surgical hypothyroidism who were treated with levothyroxine alone experienced a non-significant increase in body weight, while those who were treated with a combination of levothyroxine and liothyronine did not experience a significant change in body weight 7.

Factors Influencing Weight Gain

• Pre-existing obesity, Graves' disease, and prior weight loss have been identified as independent predictors of weight gain in patients with hyperthyroidism 4.
• The development of hypothyroidism, even transiently, has been associated with weight gain in patients with hyperthyroidism 4.
• Changes in resting energy expenditure and physical activity level have been found to correlate with changes in body weight in patients with hyperthyroidism 6.

Thyroid Medications and Weight Management

• Levothyroxine replacement therapy has been found to be effective in normalizing thyroid-stimulating hormone levels and relieving symptoms of hypothyroidism, but its effect on weight gain is unclear 3.
• The combination of levothyroxine and liothyronine has been found to prevent increases in weight and cholesterol in patients with post-surgical hypothyroidism, but more research is needed to confirm these findings 7.