What Is the Link Between Your TSH and Your Weight?

What Is the Link Between Your TSH and Your Weight?

Woman weighing herself on scale.
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Experts know that an overactive thyroid (hyperthyroidism) can cause weight loss, and a slower thyroid (hypothyroidism) may contribute to weight gain. However, the precise link between thyroid function and weight gain or loss is still fuzzy. While some experts claim the thyroid/weight link is not significant in most cases, there is some research out there to dispute this notion.

For example, a study reported in the Archives of Internal Medicine explored the connection between thyroid stimulating hormone (TSH) levels in people with normal thyroid function and body weight. The researchers found that even slight increases in TSH within the normal range, over time, appear to be associated with weight gain.

Let's take a closer look at this research and the link between TSH and your weight.

Thyroid Function and Weight

In the study mentioned above, over 2000 participants who were not receiving thyroid hormone medication and who had baseline TSH levels between 0.5 and 5.0 mIU/L (considered "normal") were examined. Their baseline TSH level and body weight were then compared to their TSH level and body weight three and a half years later.

Results revealed that modest increases in TSH in those with normal thyroid function were associated with increases in body weight in both sexes.

The study authors offered two possible reasons that might explain their findings.

First, lower energy expenditure—known as thermogenesis—is associated with lowered thyroid function. Lowered thyroid function can lower metabolism, which means that fewer calories are needed in order to maintain body weight, which can make weight loss more difficult, or even impossible in some cases.

Second, triiodothyronine (T3) is the active thyroid hormone at the cellular level. It delivers oxygen and energy to cells. Lower T3 levels are associated with lowered resting metabolic rates. And again, when metabolism is lower, lower calorie intake and more activity to burn calories are both needed in order to maintain current body weight or lose weight.

Implications

The implications of these findings aren't clear because while they suggest that increased TSH levels— even within the normal reference range—may play a role in weight gain, there is no definite cause and effect. It's not clear that lowering the TSH level (for example, with thyroid hormone replacement therapy) would reverse any impact and result in weight loss.

The main conclusion, therefore, is that while increases in TSH may be associated with weight gain, further research is needed to understand why, and whether this is preventable or treatable.

Moreover, while this study is interesting, its results are not necessarily supported by other studies. For example, a study in the Annals of Nutrition and Metabolism examined over one thousand participants with normal thyroid function and found no link between changes in TSH and body mass index after a follow-up period of ten years.

Although, this study did find a negative link between free T4 (thyroxine) and body mass index, so as free T4 levels declined over time, body mass index tended to increase.

A Word From Verywell

If you are overweight and can't lose weight despite a healthy diet and exercise, it is sensible to talk with your doctor about undergoing a thyroid evaluation to rule out hypothyroidism. An underactive thyroid can sabotage even the best diet and exercise plans.

Likewise, if you are overweight, and being treated for a thyroid problem, but finding it hard or impossible to lose weight, talk with your doctor about optimizing your thyroid treatment, and tactics thyroid patients can follow for effective weight loss

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