The snake thyroid gland: I. Seasonal variation of thyroidal and serum lodoamino acids

Abstract

The nature of thyroidal and serum iodoamino acids in two species of snakes, Naja naja and Elaphe taeniura, was examined on a seasonal basis. I −, MIT, DIT, T 3, and T 4 were found in the thyroid gland throughout the year. No attempts were made to quantify these thyroidal iodoamino acids. I −, MIT, DIT, and T 4 were found in the blood. The principal circulating iodinated substances were MIT and DIT at a concentration greater than 100 μg/100 ml serum. T 4 occurred in minute quantity only; its level as determined by resin column chromatography was about 1–2 μg/100 ml serum. This particularly low concentration of T 4 was indirectly supported by measurement of serum cholesterol level and lipoprotein concentration. There was seasonal variation in the serum T 4 level in the two species of snake. In Elaphe, T 4 level was high in the summer but it declined in late autumn and reached the lowest level in early winter. This level increased again during late winter and early spring. The reverse of this serum T 4 pattern was true in Naja. An apparent correlation existed between the thyroid activity changes and spermatogenesis in both species.