Rats were force fed a single dose of 20, 40, 80, 160, 320, 640, 1280 and 2560 μg L-5-vinyl-2-thiooxazolidone (VTO) and injected with 131I one hour later. The 131I uptake, biosynthesis of thyroid hormones and PB131I were measured 4 h after the administration of 131I. Under these conditions, a higher antithyroidal effect with VTO was found in rats fed a normal diet (ND) for 10 days before the experiment as compared with rats fed a low iodinediet (LID) for the same period. The results are described in detail. Similar doses of VTO were force fed to groups of rats fed LID daily for 20 days. The animals received 1 μCi 125I per 15 ml of drinking water for 30 days before sacrifice. One hour after the last dose of VTO they were injected with μCi 131I and killed 4 h later. In addition to the parameters presented above the thyroid weight, total 125I in the thyroid and PB125I were measured. The inhibition of thyroid activity was in direct relation to the dose of VTO administered. Finally, ethereal extracts of urine of rats kept in metabolic cages and force fed a single dose of 10 mg VTO were prepared and the dried extract was dissolved in methanol and applied to the chromatographic paper. In two chromatographic systems (1. water saturated chloroform, 2. n-butanol:acetic acid:water) two compounds were detected, one of which (Compound A) was chromatographically identical with VTO and the other (Compound B) was unknown. The Compound B was extracted from chromatograms, its UV-absorption spectrum was measured and its antithyroid activity was tested in rats. The 131I uptake was significantly inhibited following a single administration of 640 μg Compound B.

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