The influence of pinealectomy and single dose of melatonin administered at different times of day on serum T3 and T4 concentrations in rats

The influence of pinealectomy and melatonin administration on the levels of total T3 and T4 plasma levels were determined by RIA method. The results of hormonal measurements were evaluated by means of Student's t-test, and the diurnal rhythm by cosinor method. It has been established that pinealectomy disturbs the T3 diurnal rhythm and caused marked changes in T4 levels during the 24 hours. Exogenous melatonin does not influence the thyroid function during the light phase. During the dark phase, however, its inhibitory effect is prevalent in pinealectomized rats, and the stimulating in sham operated rats.