Evidence for circadian variations in serum thyrotropin, 3,5,3'-triiodothyronine, and thyroxine in the rat

We previously observed that under a 12-h light, 12-h dark schedule (lights off at 1900 h), male Sprague-Dawley rats showed a circadian rhythm for serum TSH with a zenith near midday. In the present work we further characterized the serum TSH rhythm by appropriate mathematical analysis. The peak of serum TSH occurred at 1130 +/- 0105 h (mean of five experiments), with an amplitude of 0.2 +/- 0.1 microgram/ml, while the TSH level was minimum at the beginning of the dark period; the period of the TSH rhythm was 24.5 +/- 0.6 h. A circadian rhythm was also demonstrated for serum T3 and T4; these rhythms were characterized by peaks occurring 1.5 and 2.2 h after that of TSH, respectively. As expected, characteristics of the rhythm were more narrowly defined for T3 than for T4. Serum concentrations peaked at 1256 +/- 0206 and 1346 +/- 0308 h for T3 and T4, respectively; the serum T3 concentration varied from 78.7 +/- 7.8 to 54.2 +/- 2.7 ng/100 ml (P < 0.001) and serum T4 varied from 7.3 +/- 1.1 to 5.1 +/- 0.5 microgram/100 ml (P < 0.05) for zenith and nadir values, respectively. It is suggested that the diurnal peaks of thyroid hormones might be related to TSH-induced changes in thyroid secretion.