A 76-year-old man who developed the clinical syndrome of myxedema coma was found to have a normal serum thyroxine concentration. Further investigation revealed the presence of a low serum T3 level before and after sodium levothyroxine therapy when the serum T4 value was in the hyperthyroid range. In addition, he demonstrated an inability to release TSH after intravenous administration of TRH and was found to have an enlarged sella turcica. A partial defect in peripheral conversion of T4 to T3 is postulated.