Annual variations of thyroid activity in the lizard Podarcis sicula (Squamata, Lacertidae)

The present paper describes the annual variations in plasma concentrations and thyroid and hepatic contents of thyroid hormones, as well as the activity of hepatic 5'-T4 ORD (type II) monodeiodinase, in the male lizard, Podarcis sicula, during two consecutive years. Plasma concentration of T4 was low in winter and increased in March, reaching maximal levels in May-June; plasma levels of T3 varied throughout the year, reaching minimum values in December and February. The thyroid content of T3 was high between March and July; the T4 content followed a similar seasonal pattern and showed a minimum in January and a maximum in July. Low hepatic contents of T3 and T4 were found in winter, increasing rapidly after spring to reach maximal values in July-October. Moreover, the maximal activity of hepatic 5'-T4 ORD (type II) monodeiodinase was observed in May and June-July. Minimal activity was detected in winter and in October-November. Plasma TSH levels were high between March and July and low from November to January.