Seasonal changes in thyroid activity in male and female frog, Rana perezi

Plasma T3 and T4 levels, and thyroid free (f) and bound (b) thyroid hormones contents, were determined by radioimmunoassay in adult male and female Rana perezi over 1 year. Plasma T3 levels show a significant seasonal cycle in both male and female frogs, increasing from January to peak in July (71-74 pg/ml) to decrease in August-October. Plasma T4 concentrations also varied seasonally, and sex differences were apparent. Basal T4 levels from the end of summer to the end of winter and higher values (682-651 pg/ml) in spring and early summer occur in female frogs; males have lowest levels in October (59.78 +/- 13.92 pg/ml) and higher levels during the rest of the year, except for a significant decrease in April. The thyroid content of fT3 and fT4 displayed similar seasonal patterns, with peak values in April and July-August. Sex differences are again present. The thyroid contents of bT3 are high between March and July in both sexes, with values in females being greater than those in males. The bT4 content follows a similar seasonal pattern in males and females and shows a minimum in March and a maximum in summer. Thyroid releasing capacity seems to change depending on the season and sex.