Seasonal effects on the diel changes in circulating thyroid hormones in the freshwater catfishClarias batrachus

The profiles of thyroxine (T4) and triiodothyronine (T3) in serum were monitored at 4 h intervals over a 24 h cycle in nine months of the year in females of the freshwater catfish. Clarias batrachus by means of radioimmunoassay. The results exhibited significant seasonal changes of the daily cycles in the titres of T4 and T3. Interestingly, unlike earlier reports, T4 and T3 showed two peaks in the serum; one in the photophase and the other in the scotophase of the 24 h cycle in all of the studied months. There were significant variations in their peak timings and magnitudes over the year.