Circulating thyroid hormone concentrations in relation to seasonal events in the male Indian garden lizard, Calotes versicolor

Abstract

irculating thyroxine (T₄) and triiodothyronine (T₃) were estimated by RIA in plasma samples of Calotes versicolor collected over a period of 28 months. Monthly observations on seasonal events, viz., molting, whole body O₂ consumption, abdominal fat deposition, and testes weight were also made. Plasma T₄ ranged from 0.16 to 4.63 ng/ml and T₃ from 0 to 1.76 ng/ml during the course of the year. Peaks of T₄ and T₃ could be related with prehibernating (October/November) and posthibernating (March/April) periods, when scale shedding and O₂ consumption were also enhanced. Maximum abdominal fat deposition was found in prehibernatory months (October/November). Thyroid hormone levels declined sharply with entry into hibernation. Abdominal fat deposits gradually decreased, being almost depleted by the end of hibernation. The gradual increase in thyroid hormone concentration in the later half of hibernation may be associated with increased lipolysis and preparation of the oncoming reproductive period. Although T₄ and T₃ profiles were apparently parallel, discrepancies were observed. Posthibernatory peak of T₄ was maintained much longer than that of T₃ which declined earlier. Also the rise in the prehibernatory T₃ concentration was delayed as compared to that in T₄.