Treatment of hyperthyroidism with RAI or anti-thyroid medications is associated with an increased risk of gaining weight and even developing obesity. This risk is slightly higher with RAI therapy compared to medications.

Why am I gaining weight while taking thyroid medication?

The hormones your thyroid gland releases help regulate your metabolism, or how efficiently your body burns food for energy. When your thyroid makes less of its hormones — as it does in hypothyroidism — your metabolism slows down. So you won't burn off calories as quickly and you'll gain weight.

Will I gain weight on hyperthyroid medication?

Following treatment for hyperthyroidism, patients will observe a steady weight gain despite not changes in their eating or exercise. First, the weight will approach their normal level but then many continue experience further weight gain, reaching overweight or even obesity.