Seasonal Variation in Thyroid Gland Activity in Deermice. ∗

Abstract

Summary

Thyroid gland activity as measured by percent colloid, cell height, I¹³¹ uptake and thyroxine secretion rates was studied in 2 subspecies of male deermice (P. m. bairdii and P. m. grocilis) during spring, summer, fall and winter. A seasonal difference in activity of the thyroid gland was noted in both subspecies. However, time of maximum and minimum activity also varies in the 2 subspecies. Thyroid cell height and TSR are at a maximum during December-March and a minimum in October for bairdii, and are at a maximum during October-December and a minimum during March-July for gracilis. Radioiodine uptake by the thyroid glands of both subspecies is high during July and December and low during March and October. With the exception of the October period, the thyroid gland of bairdii is more active throughout the year than the thyroid of gracilis. A lack of agreement with all parameters of thyroid activity utilized was noted.