Nonthyroidal Action of Propylthiouracil in Euthyroid, Hypothyroid and Hyperthyroid Man

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Abstract
The nonthyroidal action of 6-n-prophylthiouracil (6-PTU) on the metabolism of I131 -labeled thyroxine was investigated in patients with and without thyroid disease. Ninteen studies were performed in 15 hospitalized patients. Thyroidal radioiodine uptake was blocked with potassium perchlorate. I131-labeled T4 (thyroxine) was injected iv and its rate of disappearance was established by repeated determinations of PBI131 over 7.5 days on average. PTU was then administered and the studies were continued. Other parameters of thyroid function and metabolic status were also examined serially. The rate of T4 degradation decreased in all 5 patients with active thyrotoxicosis. One patient with euthyroid Graves'' disease, studied twice, and 2 hypothyroid patients also had decreases in thyroxine disappearance rates. There were no effects on the other study patients. Three control subjects, 2 with thyrotoxicosis, were included to define the specificity of these findings. It is concluded that 6-PTU has a nonthyroidal action in man which may be due to qualitative or quantitative differences in the metabolism of thyroid hormone, depending on thyroid status.