The snake thyroid gland: II. Radioiodine metabolism

Abstract

The pattern of the thyroidal radioiodine uptake, the nature of thyroid hormones in the thyroid gland and in the blood, and the in vitro study of the release and synthesis of iodinated substances by the thyroid gland in Elaphe were examined. The pattern of thyroidal 125I accumulation in Elaphe was similar to that of the other ophidian species, e.g., Thamnophis and Natrix, in which there was a fairly rapid uptake of 125I during the first week following the injection of 125I; a gradual decrease in the rate followed in weeks 2 and 3, and an eventual levelling-off of the thyroidal 125I radioactivity occurred. 125I-Labelled MIT, DIT, T 3, and T 4 were found in the thyroid gland 5 days after an ip injection of 125I; at this time, however no labelled iodoamino acids were detected in the blood. 125I-Labelled MIT, DIT, and T 4 were present in the circulation when measurements were made 12 days after injection of the radioiodine, and there was no appreciable change in the relative concentration of these iodoamino acids within 35 days. In vitro incubation studies of (a) a thyroid gland in the presence of 125I and (b) a thyroid gland which had been 125I-labelled for 5 days indicated that within a few hours 125I-labelled iodotyrosines were formed, and that release of MIT, DIT, and T 4 began. The possible involvement of TSH in these aspects of radioiodime metabolism in the snake thyroid gland is discussed.