April - June 2007, Volume 3, Issue 2

Ianas O, Manda D, Vladoiu S, Rosca R

The effects of melatonin treatment on circulating thyroid hormone concentration and pineal thyroxin 5'-deiodinase activity in euthyroid and hypothyroid rats

Acta Endo (Buc) 2007, 3 (2): 149-160
doi: 10.4183/aeb.2007.149

The effects of daily evening melatonin (MT) injections on plasma T3 and T4 and pineal thyroxin 5’-deiodinase (5’-D) activity in euthyroid and hypothyroid rats were investigated. Circulating levels of thyroid hormones were monitored and 5’-D activity was measured in pineal homogenates throughout the daily light-dark cycle. In the euthyroid group, T3 and T4 concentrations and pineal 5’-D activity gradually increased during the L-phase of the L/D cycle to reach maximum levels early at night. The lowest values for pineal 5’-D activity and T4 were obtained later at night when endogenous MT production was the highest. MT treatment induced an opposite circadian variation of plasma T3, T4 and pineal 5’-D activity with significant increases later at night and decreases early at night vs. the control group. In the hypothyroid group, the serum T4 and T3 concentrations significantly decreased at all moments assayed. Treatment with MT did not lead to significant changes in the propylthiouracil effect on T4 and T3 levels, but maintained the biphasic response, observed in the MT treated euthyroid group. The increases induced by PTU in pineal 5’-D activity during the light phase, were reduced from 43.61 ? 2.35 ng T3/mg protein / h to 33.36?2.87 ngT3/mg protein/h (p=0.01) by MT injections. In conclusion, the results rendered the presence of the 5’-D in the rat pineal, its activity showing a circadian pattern similar to the circulating T4 levels. The MT treatment induced an opposite circadian variation of serum T3, T4 and pineal 5’-D activity suggesting an interaction between the light/dark cycle, 5’-D activity and responsiveness to MT.

Keywords: pineal gland, melatonin, thyroxin 5’-deiodinase, thyroid hormones

Correspondence: Olga Ianas, PhD, "C.I.Parhon" Institute of Endocrinology, Bd.Aviatorilor, 34-36, 011863, Bucharest, Romania, e-mail: oianas@yahoo.com