Interaction of ageing, photoperiod and melatonin on plasma thyroid hormones and cholesterol levels in female Syrian hamsters (Mesocricetus auratus).

Plasma levels of thyroxine (T4), triiodothyronine (T3) and growth hormone (GH) as well as the free T4 and free T3 index (FT4I, FT3I) in 20-month-old female Syrian hamsters kept under long daily photoperiod [light:dark (LD) cycles of 14:10] were significantly depressed compared to the values of 2-month-old animals kept under the same photoperiodic conditions. Cholesterol levels in old females were significantly elevated compared to those in young animals. In a subsequent experiment, young (4 month) and old (15 month) female hamsters were either placed in a short photoperiod (10:14 LD) or a 14:10 LD cycle and given daily afternoon injection of either diluent or 25 micrograms melatonin. After 8 weeks, the old females, regardless of treatment or photoperiod, had significantly lower T4 levels and higher cholesterol and triglyceride levels than did young animals. In 14:10 LD, FT4I and FT3I did not differ between young and old animals, possibly because of the reduced age gap in this experiment. Although the FT4I and FT3I in young females after 8 weeks of treatment with 10:14 LD or melatonin were not significant, the old females did demonstrate a significant suppression of their FT4I and FT3I values.