SUMMARY

When potassium [35S]thiocyanate was administered to rats, treatment with thyroid-stimulating hormone (TSH) diminished the amount of 35S per unit weight of thyroid, as well as the thyroid: plasma 35S ratio (T: P) for compounds containing 35S. Chromatographic analysis of thyroid gland homogenates and plasma showed that under the influence ofTSH the 35SCN\x=req-\ fraction increased, while the 35SO42\m=-\fraction decreased in the thyroid. The T:P ratio calculated for both fractions confirmed the increase of the 35SCN\x=req-\ concentration in the thyroid. Chromatographic analysis ofthe plasma showed that the 35SCN\m=-\fraction in the TSH-treated animals increased gradually while it decreased in the control group. Experiments with guinea-pig thyroid lobes in vitro agreed with the results in vivo. A significant increase of 35SCN\m=-\and a decrease in 35SO42\m=-\ were found in TSH-treated thyroids after 6 h of incubation. Even clearer results were obtained by analysis of the medium. Oxidation of [35S]thiocyan-ate to [35S]sulphate, with a simultaneous release into the medium of the latter, took place only in the control thyroid lobes. In the TSH-treated thyroids, no changes were found in the percentage content of 35S compounds in the medium during 6 h of incubation. These results suggest that TSH inhibits the degradation of thiocyanate in the thyroid