A STUDY OF THE MECHANISM OF THE INHIBITION OF THE THYROID GLAND INDUCED BY INGESTION OF THYROID SUBSTANCE*†

Abstract

THE ingestion of thyroid hormone depresses the activity of the thyroid gland. Farquharson and Squires (1) reported that, upon cessation of ingestion of thyroid substance by normal individuals, the basal metabolic rate (BMR) fell to below pretreatment control levels. In a myxedematous patient, the BMR fell to pretreatment levels and not below. Riggs et al. (2) and Winkler et al. (3) illustrated the same phenomenon by demonstrating a fall of serum protein-bound iodine (PBI) to below normal levels in euthyroid subjects after discontinuance of thyroid medication. Recently, Greer (4) found that the uptake of radioactive iodine (I131 uptake) by the thyroid gland of normal persons was markedly inhibited by the ingestion of desiccated thyroid; it returned to normal within four weeks after cessation of the thyroid ingestion. He noted that the daily ingestion of 60–180 mg. of thyroid depressed the I131 uptake in over 90 per cent of his subjects. Last year, a patient with a functioning struma ovarii was studied in this laboratory (5). Serial studies of her serum PBI levels and of the I131 uptake by the cervical thyroid gland suggested that the secretions of the struma ovarii had depressed the function of the cervical thyroid gland.