Correlation of plasma propylthiouracil levels with inhibition of thyroid hormone synthesis in the rat

The inhibition of thyroid hormone synthesis was studied in relation to plasma levels of 6-n-propyl-2-thiouracil (PTU). Na ¹²⁵I (5 μCi) was injected i.p. into adult male Sprague-Dawley rats. After 30 min, graded doses of PTU (0.2 mg/kg, 0.1 mg/kg, and 0.05 mg/kg) were similarly injected. Thyroid hormone synthesis was followed by the accumulation of radioactivity into thyroid glands, which were removed at specified time intervals. PTU levels were measured spectrophotometrically at the time of sacrifice. Additionally, PTU (³⁵S) was used to confirm blood levels of PTU and also to follow intrathyroidal PTU levels. Plasma PTU levels in excess of 0.18 μg/ml completely inhibited thyroid hormone synthesis. Levels between 0.14 and 0.09 μg/ml had a partial effect, and PTU levels less than 0.09 μg/ml had no effect on thyroid hormone synthesis.