A middle-aged female became hypothyroid seven years after the onset of diffuse toxic goiter and 1½ years after a third course of propylthiouracil. The thyroid was not palpable. No long-acting thyroid stimulator (LATS) or abnormal thyroid antibodies were detected. Serum thyroid-stimulating hormone (TSH) was moderately elevated and other pituitary hormones were intact. TSH stimulation tests on two occasions produced complete and remarkably sustained clinical and laboratory remissions lasting from three to six months. After five years of thyroid replacement all medication was stopped. The serum thyroxine fell to 0.5 µg/100ml, the serum TSH rose to 81 µU/ml, and there was no longer any response to TSH stimulation. The final stage of glandular failure was thus preceded by an intermediate stage with features of both primary and secondary hypothyroidism and an unusual responsiveness to exogenous TSH.