OBJECTIVE Patients frequently express concern that treating hyperthyroidism will lead to excessive weight gain. This study aimed to determine the extent of, and risk factors for, weight gain in an unselected group of hyperthyroid patients.
DESIGN and SUBJECTS We investigated 162 consecutive hyperthyroid patients followed for at least 6 months. Height, weight, clinical features, biochemistry and management were recorded at each clinic visit.
RESULTS Documented weight gain was 5·42 ± 0·46 kg (mean ± SE) and increase in BMI was 8·49 ± 0·71%, over a mean 24·2 ± 1·6 months. Pre-existing obesity, Graves' disease causing hyperthyroidism, weight loss before presentation and length of follow-up each independently predicted weight gain. Patients treated with thionamides or radioiodine gained a similar amount of weight (thionamides, n = 87, 5·16 ± 0·63 kg vs. radioiodine, n = 62, 4·75 ± 0·57 kg, P = 0·645), but patients who underwent thyroidectomy (n = 13) gained more weight (10·27 ± 2·56 kg vs. others, P = 0·007). Development of hypothyroidism (even transiently) was associated with weight gain (never hypothyroid, n = 102, 4·57 ± 0·52 kg, transiently hypothyroid, n = 29, 5·37 ± 0·85 kg, on T4, n = 31, 8·06 ± 1·42 kg, P = 0·014). This difference remained after correcting for length of follow-up. In the whole cohort, weight increased by 3·95 ± 0·40 kg at 1 year (n = 144) to 9·91 ± 1·62 kg after 4 years (n = 27) (P = 0·008), representing a mean weight gain of 3·66 ± 0·44 kg/year.
CONCLUSION We have demonstrated marked weight gain after treatment of hyperthyroidism. Pre-existing obesity, a diagnosis of Graves' disease and prior weight loss independently predicted weight gain and weight continued to rise with time. Patients who became hypothyroid, despite T4 replacement, gained most weight.