Objective: 

Although there is an abundance of reports on the relationship between the pineal gland and thyroid hormones, there is little information in the literature about the role of the melatonin in thyroid dysfunction. The present study aims to examine the effect of pinealectomy and/or melatonin administration on leptin and NPY secretion in hypothyroidism induced rats.

Methods: 

The study included 50 adult male Sprague-Dawley rats which were allocated into 5 groups: Group 1, control; group 2, sham-hypothyroidism; group 3, hypothyroidism (group which was injected 10 mg/kg/day ip PTU for 4 weeks); group 4, hypothyroidism + pinealectomy group (the group in which the pineal gland was removed under general anesthesia and which was injected 10 mg/kg/day ip PTU for 4 weeks); and group 5, hypothyroidism + melatonin administered group (the group which was injected 10 mg/kg/day ip PTU and 3 mg/kg/day melatonin for 4 weeks). At the end of the 4-week procedures, the rats were decapitated and blood samples were obtained. Plasma levels of melatonin, leptin and NPY were determined by RIA.

Results: 

The highest melatonin levels were obtained in the melatonin-administered group 5 and the lowest levels were found in the pinealectomy group (group 4) in the study (p<0.001). Melatonin levels in the hypothyroidism group (group 3) were higher than those in group 4, but significantly lower than the levels in all other groups (p<0.001). Experimental hypothyroidism resulted in an increase in leptin and neuropeptide-Y levels in all groups (groups 3, 4 and 5).

Conclusions: 

Results of the present study indicate that hypothyroidism causes significant inhibition in melatonin levels and a remarkable elevation in leptin and NPY levels. We conclude that the increase in leptin and NPY levels is a result of hypothyroidism, rather than from pinealectomy and/or melatonin administration.