Accumulation of 35S-thiouracil by the rat thyroid gland

Large and small doses of 35S-thiouracil were administered ip to 2 groups of rats and the distribution and metabolism of 35S-radioactivity in the thyroid and plasma studied for 48 hr. With both doses, significant thyroid to plasma ratios >1 were obtained for both total 35S-radioactivity and residual unmetabolized 35S-thiouracil. However, with the smaller dose, the accumulation of the 35S-label was much more rapid and higher thyroid to plasma 35S-ratios were obtained throughout. These results suggested that the accumulation of thiouracil by the thyroid is dose dependent. Five 35S-compounds were found in the thyroid by thin-layer chromatography: protein-bound radioactivity, sulphate, 2 unknown compounds and unmetabolized thiouracil. Similar 35S-compounds were found in the plasma. (Endocrinology93: 162, 1973)