We describe the case of a 49-y-old female patient on long-term parenteral nutrition after abdominal surgery who failed to gain weight despite nutritional provision in excess of theoretical requirements. On investigation, she was found to have a negative nitrogen balance (-5.9 g) and to have a tri-iodothyronine thyrotoxicosis but without many of the typical clinical features of hyperthyroidism. The patient also had mild hypercalcemia and hyperphosphatemia, which resolved fully after mobilization and treatment of the thyrotoxicosis. A derangement of the liver function tests was observed, which worsened progressively during parenteral nutrition but resolved promptly at its discontinuation. This case illustrates the importance of carrying out appropriate investigations including all thyroid function tests on patients who fail to gain weight on nutritional support.