Propylthiouracil (PTU) Pharmacology in the Rat,II. Effects of PTU on Thyroid Function*

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Abstract
Using a sensitive and specific radioimmunoassay for propylthiouracil (PTU), the effects of short term (1 wk) and long term (1 mo.) PTU treatment on thyroid function were examined in the rat, and changes in thyroid function correlated with serum and thyroid PTU levels. After 1 wk, dose-dependent decreases in thyroid PBI [protein bound iodine], serum T4 [thyroxine] and serum T3 [triiodothyronine] were observed, with concomitant elevations in the serum reverse T3 to T4 ratio and serum TSH. Fifty percent suppression of thyroid PBI occurred at a PTU concentration in the drinking water of 0.0005% (ED50), with concomitant serum and thyroid PTU levels of 0.3 .mu.g/ml and 300 ng/thyroid, respectively. After 1 mo. of PTU, serum T4 values were lower than after 1 wk of treatment for all PTU concentrations, but values for the other thyroid functional variables were similar to those in the 1 wk group at comparable PTU dosage. The PTU dose-response curve for thyroid PBI was similar to that seen after 1 wk of treatment, with an ED50 of 0.0004%. After discontinuation of PTU treatment, PTU disappeared from serum in a biexponential fashion, with an early rapid distribution phase (t1/2 = .apprx. 4 h) and a 2nd slower elimination phase (t1/2 = .apprx. 2.6 days). In the thyroid, an initial increase in PTU content was seen up to 18 h after PTU withdrawal; thereafter, thyroid PTU declined linearly, with a t1/2 of 1.4 days in both groups. After PTU withdrawal, thyroid PBI recovered with a t1/2 of 1.09 days after 1 wk on PTU, but recovery was prolonged (t1/2 = 2.8 days) after 1 mo. of treatment. Log thyroid PTU and log thyroid PBI were linearly related after PTU withdrawal (r = 0.97; P < 0.001) after 1 wk but not after 1 mo. Serum T4 and serum T3 remained below control values for 2 days, but then rapidly normalized, with T3 values rising transiently above the control value. This rebound occurred at a time when PTU was still present within the thyroid, before thyroid PBI had returned to baseline. These data indicate a close inverse relationship between PTU dose and both thyroid hormone biosynthesis and peripheral T4 deiodination. In addition, short and long term PTU treatments have quantitatively similar effects on thyroid function, although recovery of thyroid function is prolonged after long term treatment. The biexponential disappearance of PTU from the serum is compatible with a 2-compartment model of PTU distribution. The early increase in thyroid PTU after drug withdrawal is suggestive of an inhibitory effect of PTU upon its own uptake by the thyroid, whereas the faster disappearance of PTU from the thyroid than from serum is consistent with intrathyroid drug metabolism.