Basal metabolic rate (BMR) used to be an important test for analyzing patients’ thyroid status. Patients with underactive thyroid glands have low BMRs, and vice versa.

Due to the complexity of measuring BMR, this practice fell out of favor. Various other mechanisms likely play a role in the relationship between thyroid disease, body weight, and metabolism (the amount of oxygen used by the body during a period of time).

 

Autoimmune disease and weight

Studies have pointed to a relationship between thyroid autoimmunity and obesity. The proposed relationship could be bidirectional, with some data suggesting that thyroid autoimmunity is the cause of hyperlipidemia and abdominal obesity, while others indicate it’s a result of obesity.

According to authors of an article published in Endocrine Research, the adipocytokine leptin could play a pivotal role in the development of thyroid autoimmunity in obese individuals.

Amouzegar A, Kazemian E, Abdi H, et al. Abdominal obesity phenotypes and incidence of thyroid autoimmunity: A 9-year follow-up. Endocr Res. 2020;45(5):1–8.

 

“It is argued that obesity can impair thyroid function in different aspects such as activation of the hypothalamic–pituitary–thyroid axis and increased deiodinase activity,” the authors wrote. “Changes in thyroid function tests in obese individuals are probably a result of obesity rather than its cause, since weight loss normalizes thyroid function tests.”

 

Weight and thyroid cancer

In a study published in Scientific Reports, Korean researchers mined a large population-based cohort to assess the relationship between obesity and thyroid cancer.

Kwon H, Han KD, Park CY. Weight change is significantly associated with risk of thyroid cancer: A nationwide population-based cohort study. Sci Rep. 2019;9(1):1546.

The investigators excluded participants who were receiving treatment for hypothyroidism or hyperthyroidism.

 

They found that higher BMI and larger waist circumference correlated with increased risk of thyroid cancers in both men and women, with higher BMI and larger waist circumference showing an even more pronounced association in men.

Investigators also found that weight gain in lean individuals increased the risk of developing thyroid cancer, whereas weight loss in obese individuals decreased that risk.

Explanations tying obesity and thyroid cancer could include inflammation, impaired immune response, oxidative stress, higher thyroid stimulating hormone (TSH) levels, hyperinsulinemia, adipokines, and higher aromatase activity. Chronic low-grade inflammation could also result in the formation of reactive oxygen species, lower tumor suppression, and a higher cell-cycle rate.

Furthermore, higher TSH levels could trigger the growth of thyroid cells with higher levels of mutation, leading to the proliferation of thyroid cancer.

 

Weight changes after treatment

Patients who receive treatment for hyperthyroidism can expect to gain back any weight they originally lost due to the condition, according to the American Thyroid Association.

Thyroid and weight. American Thyroid Association. 2022.

Even if they never lost weight, they may gain some upon starting treatment because they’ve grown accustomed to eating more calories while hyperthyroid.

 

On the other hand, hypothyroid patients tend to lose some weight during treatment. Because most weight gained secondary to hypothyroidism is due to accumulation of salt and water, a patient’s expected weight loss is usually 10% or less of their body weight. The usual weight loss seen in hypothyroidism is relatively less than the weight gain seen with this condition. It is rare to see massive weight gain in hypothyroid conditions.

Treatment for hypothyroidism results in a return to the baseline weight maintained before the condition began. Once hypothyroidism is treated, the ability to gain or lose weight is the same as in any euthyroid individual.

Clinicians should keep in mind that if weight gain is the only symptom indicative of hypothyroidism, then it’s less likely that weight gain is due only to an underactive thyroid.

Although excessively increasing thyroid levels can lead to more weight loss than dieting alone, the practice carries risks such as loss of muscle protein and bone, and heart problems. And once the administration of excess thyroid hormone is stopped, any lost weight is usually regained.