Circadian oscillations of thyroid hormones, insulin and glucagon in the blood of laboratory rats in the course of the year.

The concentration of thyroid hormones and insulin in the serum that of glucagon in the plasma and glucose in the blood was determined at 3-hour intervals, in the course of one day, at different times of the year, in adult male rats (Wistar strain) of a conventional breed kept under standard conditions with a 12:12 h light:dark regimen. The lowest thyroid hormone and glucagon concentrations and the highest insulin and blood glucose levels were found in the winter. In various seasons, circadian oscillations of the thyroid hormones culminated in the dark part of the day and that of glucagon in the light part, with the exception of the autumn. Circadian oscillation of insulin levels culminated at different times of day during the year. The pronounced changes found in the examined hormones in the laboratory rat at various times of the year are evidently the outcome of adaptation to changes in external environmental conditions during phylogenesis. In the polarity of changes in these indicators between the winter and the summer or the spring, the laboratory rat bears the closest resemblance to wild mammals.