The effect of increased dark phase of photoperiodic cycle on serum level of T3, T4 and TSH in male rats

Studies have shown that alteration in photoperiod influences secretion of many hormones. The aim of this study was to investigate the effect of different periods of darkness on serum level of T3، T4 and TSH in male rats.

In this experimental study، 30 male Wistar rats were randomly divided into control group (exposed to 12 h light-12 h dark)، and groups exposed to 13، 15، 21 or 21 h of darkness/day of 6 rats in each group. After 8 weeks، blood samples were collected using cardiac puncture method and following serum collection، the levels of T3، T4 and TSH were measured by radioimmunoassay. The data were statistically analyzed using ANOVA.

Serum T3 and T4 levels did not significantly change in rats exposed to darkness for 13 and 15 h/day as compared to control animals; however، significantly decreased in rats exposed to darkness for 18 and 21 h/day (p<0. 05). Serum level of TSH did not significantly change in experimental groups as compared to control animals.

The findings suggest that prolonged periods of artificial darkness at least up to 50% over the standard scale can reduce thyroid activity and serum levels of thyroid hormone. It is suggested that darkness can be considered as a probable factor in hypothyroidism occurrence.