Diurnal Variation of Plasma and Pituitary Thyrotropin (TSH) of Rats.

Abstract

The diurnal variation in TSH content of the plasma and pituitary of the female adult rat under conditions of 14 hr. of light and 10 hr. of darkness was determined by an immunochemical technique. The plasma TSH level rose gradually from 3 A.M. to 3 P.M., then declined; whereas the pituitary TSH was highest at 3 A. M. and then declined until 3 P. M. These data suggest that the thyroid hormones would be lowest some time prior to 3 A. M. and reach a peak some time after 3 P. M., thus stimulating maximal activity during the night.