Patients Treated for Hyperthyroidism Are at Increased Risk of Becoming Obese: Findings from a Large Prospective Secondary Care Cohort.

Background: The most commonly reported symptom of hyperthyroidism is weight loss; successful treatment increases weight. Weight gain faced by patients with hyperthyroidism is widely considered a simple reaccumulation of premorbid weight, whereas many patients feel there is a significant weight "overshoot" attributable to the treatment. We aimed to establish if weight gain seen following treatment for hyperthyroidism represents replenishment of premorbid weight or "overshoot" beyond expected regain and, if there is excessive weight gain, whether this is associated with the applied treatment modality. Methods: We calculated the risk of becoming obese (body mass index [BMI] >30 kg/m²) following treatment for hyperthyroidism by comparing BMI of 1373 patients with overt hyperthyroidism seen in a secondary care setting with the age- and sex-matched background population (Health Survey for England, 2007-2009). Next, we investigated the effect of treatment with an antithyroid drug (ATD) alone in regard to ATD with radioactive iodine (¹³¹I) therapy. We modeled the longitudinal weight data in relation to the treatment pathway to thyroid function and the need for long-term thyroxine replacement. Results: During treatment of hyperthyroidism, men gained 8.0 kg (standard deviation ±7.5) and women 5.5 kg (±6.8). At discharge, there was a significantly increased risk of obesity in male (odds ratio = 1.7 [95% confidence interval 1.3-2.2], p < 0.001) and female (1.3, 1.2-1.5, p < 0.001) patients with hyperthyroidism compared with the background population. Treatment with ¹³¹I was associated with additional weight gain (0.6 kg, 0.4-0.8, p < 0.001), compared with ATD treatment alone. More weight gain was seen if serum thyrotropin (TSH) was markedly increased (TSH >10 mIU/L; 0.5 kg, 0.3-0.7, p < 0.001) or free thyroxine (fT4) was reduced (fT4 ≤ 10 pmol/L (0.8 ng/dL); 0.3 kg, 0.1-0.4, p < 0.001) during follow-up. Initiation of levothyroxine was associated with further weight gain (0.4 kg, 0.2-0.6, p < 0.001) and the predicted excess weight gain in ¹³¹I-induced hypothyroidism was 1.8 kg. Conclusions: Treatment for hyperthyroidism is associated with significant risks of becoming obese. ¹³¹I treatment and subsequent development of hypothyroidism were associated with small but significant amounts of excess weight gain compared with ATD alone. We advocate that the discussion over the weight "overshoot" risk forms part of the individualized treatment decision-making process.