Studies were carried out in a hyperthyroid patient before, during, and after hypophysectomy as follows: (1) Levels of protein-bound iodine (PBI), butanol-extractable iodine serum (BEI), cholesterol, and131I uptake were the clinical measurements chosen to evaluate the function of the thyroid gland. (2) Metabolic studies were conducted after the administration of thyroxine I 131 and sodium iodide I 131. (3) Levels of thyrotropic hormone (TSH) and long-acting thyroid stimulator (LATS) in the serum provided information about pituitary function. (4) Detailed histopathologic studies of the brain, midbrain, pituitary, pituitary stalk, median eminence, and thyroid gland were performed. (5) Serial sections of the sella turcica revealed that the hypophysectomy was total. This study revealed that the thyroid gland of the patient was able to increase the131I uptake, maintain a normal PBI level and incorporate131I into organ systems several months after hypophysectomy. Chromatographic studies of the pituitary gland after the administration of thyroxine I 131 showed iodine, tetraiodothyronine and triiodothyronine. The median eminence and hypothalamic areas were histologically normal.