Daily changes in thyroid activity in the frog Rana perezi: variation with season

Plasma triiodothyronine (T3) and thyroxine (T4) levels, as well as thyroid free (f) and bound (b) thyroid hormones (TH) content, were determined by radioimmunoassay (RIA) in adult Rana perezi frogs during a 24 h cycle in winter, spring, summer, and autumn. Significant daily changes in plasma T3 levels were present in all the seasons except for winter, being the lowest values observed during the scotophase. In contrast, plasma T4 only showed significant changes in spring, following a similar pattern to the one described for T3. Thyroid fT3 content did present day/night significant changes only in spring showing high contents at early scotophase. Mean fT4 content was higher at the beginning of light phase than during the rest of daily photocycle in spring and autumn, but significant differences appeared only in autumn. Regarding the thyroid bound content of TH, bT3, and bT4 presented significant daily changes in spring and autumn. However, different profiles were observed in these two seasons. High bound contents were found at early photo- and scotophase with lower values at late dark phase in spring, whereas higher contents were detected at this time in autumn. The present results indicate the existence of seasonally changing daily fluctuations in thyroid activity in Rana perezi and it seems that an interaction between photoperiod and temperature plays a role in the regulation of these daily changes.