Can a Thyroid Hormone Reverse Weight Gain from Hypothyroidism?

When patients with primary hypothyroidism report weight gain, most clinicians understandably believe that initiating appropriate thyroid hormone treatment will reverse the symptom. However, researchers at the Boston University School of Medicine noted that only a few small studies have examined the issue of weight change in hypothyroid patients following levothyroxine (LT4) treatment.

To address this void, the researchers examined the medical records of adults with newly diagnosed primary hypothyroidism. Their retrospective cohort study published in Endocrine Practice included 101 patients who were treated with LT4 for initial thyroid-stimulating hormone (TSH) ≥10 mIU/L.

In addition to recording TSH levels and weights at the time of hypothyroidism diagnosis and up to 24 months after LT4 initiation, the investigators measured the patients’ weights at the first visit when their post-treatment serum TSH fell below 5 mIU/L.

Although the initial median TSH measurement was 18.3 mIU/L, it decreased to 2.3 mIU/L after treatment, which indicated that LT4 worked as expected. At diagnosis, the patients’ median weight was 79.6 kg and median body mass index (BMI) was 29.3. At approximately 5 months, the median change in weight was -0.1 kg, while the median change in BMI was -0.1 kg/m².

Since only half of the patients lost weight, and no demographic factor predicted successful changes in weight or BMI, the researchers concluded the popular belief that LT4 therapy in primary hypothyroidism results in weight loss is incorrect.