Objective: Individuals with hyperthyroidism lose weight despite increased appetite and food intake, and weight is regained after treatment of hyperthyroidism. We asked whether this weight regain is purely a function of lowered metabolic rate coincident with lowered thyroid hormone concentrations or if the weight gain is related to food-energy overconsumption.
Methods: Ten unselected patients with hyperthyroidism treated with 131I were studied. The following measurements were made at 0, 1, 2, 3, 6, and 12 months: total food energy, carbohydrate, fat and protein consumption; serum thyroxine (T4); serum triiodothyronine (T3); T3 resin uptake; serum thyroid stimulating hormone (TSH); weight; height; and 24-hour urinary urea excretion.
Results: Inverse changes in body weight and food energy consumption/kg throughout the period of observation was a striking finding (mean initial weight 67.1+/-5 kg, final weight 76.4 kg+/-3 kg, premorbid weight 77.1+/-5 kg). The initial and final food energy intake was 3005+/-199 and 2597+/-137 Kcal/24 hrs, respectively. The thyroid hormone concentrations declined inversely relative to weight gain during the first months of the study, but later the thyroid hormones increased while weight gain continued. Initial serum T4 15.0+/-1 value at three months was 4.0+/-1.0 mg/dl, final T4 11.0+/-1.
Conclusion: We conclude that weight gain following treatment of hyperthyroidism is due to 1) reduction in metabolic rate consequent upon the decreased thyroid hormone concentrations and 2) food energy intake which was initially greater than required to maintain individuals' premorbid weight. As body weight increased, food intake declined and both reached an asymptotic limit.