Obesity is a global health concern and the proportion of overweight and obese people in the UK is rapidly increasing. Patients undergoing treatment for hyperthyroidism frequently express concerns regarding excessive weight gain, especially when offered treatment with I-131. We investigated 1047 patients with overt hyperthyroidism to determine the extent of weight changes and to identify risk factors for weight gain following treatment. Overall, 727 (69.4%) patients gained ≥5% of their presenting body mass; the mean gain was 9.9±0.2 kg, resulting in a 3.6±0.08 kg/m2 BMI increase during 22.03±0.42 months. 44.2% of patients with normal BMI at presentation became overweight or obese and 44.6% of overweight patients developed obesity at the end of treatment. Weight gain was most intense during the initial 6 months of treatment but continued relentlessly during 36 months when body weight had increased >10% (P<0.001). At discharge, the proportions of obese men and women were significantly higher when compared with age and gender specific population of West Midlands (P<0.001). Similar proportions of subjects gaining weight were observed in those treated with thionamides and subjects receiving one or multiple doses of I-131 (weight gain in 66.8%, 70.5%, 72.3% respectively, P=NS). The reporting of weight loss prior to presentation (n=702, AOR:3.0, P<0.001), higher presenting serum fT4 concentrations (AOR:1.01 per 1 pmol/l, P<0.001), longer treatment duration (AOR:1.02 per month, P=0.001), and male gender (n=226, AOR:1.44, P=0.05) were independently associated with increased probabilities of weight gain. Patients age, smoking status, ethnic origin, weight status at presentation, the treatment modality and the development of hypothyroidism were not associated with increased likelihoods of weight gain.
Conclusion: Cure of hyperthyroidism is associated with marked weight gain and significantly increased risks of becoming overweight or obese, independent of the treatment modality employed. Subjects with more severe hyperthyroidism, those reporting prior weight, subjects requiring longer treatment and men are particularly at risk of gaining weight.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: No specific grant from any funding agency in the public, commercial or not-for-profit sector.