Seasonality in thyroid function in chalcides ocellatus (reptilia, scincidae)

Abstract

To characterize seasonal changes in thyroid function in a terrestrial reptile, Chalcides ocellatus, variations in thyroid gland, in plasma levels of thyroid hormones, 3, 5, 3'triiodothyronine (T3) and thyroxine (T4), thyroid stimulating hormone (TSH), in hepatic contents of thyroid hormones, and 5'-T4 (ORD) (type II) monodeiodinase activity were measured over a period of two years. Morphologically, the thyroid gland exhibited distinct cycles, with a reduction of activity in November, a complete stop in December- January (low follicular epithelium, colloid compact and devoid of reabsorption vacuoles), a fair secretory activity in March- April and a maximum of activity in June-July (follicular epithelium very high and colloid retracted, with clear signs of reabsorption). At the same time, plasma levels of T3 and T4 were low in winter but increased in March, reaching maximal levels in July. A low hepatic content of thyroid hormones was found during the winter, but it increased rapidly after spring to maximal values in July. Besides, the maximal activity of the hepatic 5'-T4 ORD (type II) monodeiodinase was recorded in July, while minimal activity was present in winter. Plasma TSH levels were high between March and July and were low from November to January.