Gastric bypass is increasingly used in morbidly obese patients to achieve significant reduction of body weight and fat mass and concurrent improvement in co-morbidities. We report the case of a 53-year-old male patient (141 kg, BMI 50 kg/m2), successfully treated by amiodarone for supraventricular arrythmia, who underwent Roux-en-Y gastric bypass (RYGBP). 6 months after surgery, he had lost 45% of his preoperative weight (44.8% of weight loss was lean mass) and developed amiodarone-induced subclinical hyperthyroidism. We hypothesize the following sequence of events: weight loss after RYGBP, therefore fat loss, decrease in distribution volume of amiodarone inducing iodine overload and hyperthyroidism, reinforcing weight loss and particularly loss of lean mass. This report emphasizes the importance of careful monitoring of weight and body composition changes after RYGBP. In this situation, checking thyroid status is recommended, especially when there is a history of thyroid disease or potentially toxic thyroid medication.