Thyroid hormones in male hamsters with activated pineals or melatonin treatment.

Blinding resulted in gonadal and prostatic atrophy and reduced plasma thyroxine (T4), free T4 index (FT4I) and reverse triiodothyronine (rT3) levels in adult male hamsters housed in light-to-dark, 14:10 h. Similar effects were seen after daily evening injections of 25 microgram melatonin. Pinealectomy prevented the effects of blinding or melatonin injections. There were no pineal- or melatonin-induced decrements in T3 or thyrotrophin (TSH) concentrations. TSH was elevated by blinding in one experiment but not in another, despite suppression of T4 and FT4I in both. Orally administered melatonin (approximately 245 microgram daily in drinking water through the evening and night) reduced the weight of testes and prostates and slightly lowered plasma T4 and FT4I, indicating the effectiveness of melatonin by this route. The capability of the pineal and of melatonin to suppress plasma T4 is not a result of sex-steroid-induced alteration of plasma binding but is most likely a result of variable suppression of the pituitary-thyroid axis at the level of TSH regulation and also at the level of T4 secretion and/or metabolism. Reduced rT3, but not T3 levels after blinding, may reflect the pineal-induced deficit in T4 as a substrate for rT3 formation, altered peripheral conversion of T4 or altered disposal of thyroid hormones.