The function of thyroid glands of ground squirrels, maintained under conditions of constant illumination and temperature as measured by the release of radioiodine (125I), varied from one time of the year to the next and differed among species. The maximum release rate was 2.476% per day and was observed in the high altitude species, Spermophilus lateralis, while the slowest release rate was 0.029% per day and was observed in the desert species, S. townsendi. The periods of slowest release tended to coincide with late summer and early fall, though individual animals of the eight species in which 125I release rates were studied varied considerably among themselves. The variation within species was thought to reflect the free-running circannual rhythm of thyroid secretion which eventually became out of phase with natural conditions. The reason for differences among species is unknown, though it may be related to water balance. The rhythm of thyroid activity is not a function of body temperature rhythms, but ...

Vijayan E

Papers

Histophysiological changes in the thyroid gland of the Indian palm squirrel Funambulus pennanti (Wrouhton) in response to varied doses of radioiodine.