Studies in nonmyxedematous hypometabolism. II. Turnover of I131-labeld thyroxine after intravenous infusion.

Abstract

THE term “euthyroid hypometabolism,” has been utilized to describe a group of patients having basal metabolic rates below −20 per cent in association with a variety of clinical symptoms suggestive of hypothyroidism but in whom other customary parameters of thyroid function are within normal limits. The concept that the low metabolic rate might be the result of a defect in the peripheral metabolism of thyroxine received support from our experience in the treatment of such patients with triiodothyronine or with mixtures of triiodothyronine and thyroxine (1, 2). In each of 4 cases, the basal metabolic rate was persistently elevated following the oral administration of triiodothyronine, alone or with thyroxine, although equivalent doses of desiccated thyroid had been ineffective. Previous studies in such patients failed to reveal any abnormality of thyroidal function as measured by the serum levels of protein-bound iodine and cholesterol, or by the 24-hour thyroidal uptake of I131. With the availability of I131-labeled l-thyroxine, it became possible to investigate more directly the peripheral metabolism of thyroxine. The utility of this technique has been demonstrated (3, 4) in a small number of subjects with normal thyroid function, hyperthyroidism, myxedema, panhypopituitarism and hypermetabolism without thyroid disease, but no such data are available in patients with euthyroid hypometabolism. The present communication reports the results of preliminaiy experiments designed to study the peripheral metabolism of thyroid hormone in patients with this syndrome.