Free thyroxin index in normal, melatonin-treated and blind hamsters.

The administration of melatonin to young male hamsters depressed thyroxin levels, as measured by radioimmunoassay, by 38.5% of saline controls. Free thyroxin index was also signficantly depressed by melatonin injections. The depression of thyroxin and free thyroxin index by melatonin injections was more effective in intact than in pinealectomized animals. Blinding of hamsters also reduced both thyroxin and free thyroxin index.