THE USE OF THYROTROPHIN IN THE DIFFERENTIAL DIAGNOSIS OF PRIMARY AND SECONDARY HYPOTHYROIDISM

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Abstract
The functional state of the thyroid was studied prior to and after admn. of thyrotropic hormone in 10 euthyroid patients, 10 with primary myxedema, and 8 with hypopituitarism. Urinary excretion of I131, extra-renal disposal rate of I131 "thyroid collection rate", the serum protein-bound I, and basal metabolic rate were used as indices of thyroid function. Thyrotrophin (TSH) was admn. for 6 days or more. In the euthyroid subjects the urinary excretion of I131 decreased and the extra-renal disposal rate of I131 increased markedly. Following admn. of TSH the values were consistent with thyroid hyperfunction. Serum protein-bound I also increased significantly. In patients with primary thyroid myxedema the urinary excretion of I131, extra-renal disposal rate of I131, and serum protein-bound I values did not change significantly. In patients with hypopituitarism with secondary hypothyroidism the urinary excretion of I131 decreased, extra-renal disposal rate of I131 increased, and serum protein-bound I values increased. The rise in these indices of thyroid function was comparable to the increase observed in euthyroid subjects, although smaller in most cases. Results suggest that stimulation of the thyroid with thyrotropic hormone may be used as an aid in distinguishing between primary and secondary hypothyroidism.