Melatonin-induced decrease of the size of thyrocyte nuclei in rat thyroids incubated in vitro

The mean volume, intersection area and perimeter of thyrocyte nuclei in rat thyroid lobes incubated with melatonin (Mel), thyrotropin (TSH) or with both substances together, were investigated. The measurements were carried out using a computer system for karyometric examination. Melatonin in concentrations of 10(-8) M, 10(-10) M and 10(-12) M significantly decreased the mean nuclear volume and intersection area of thyrocytes (p < 0.01); the mean perimeter of thyrocyte nuclei was significantly decreased by melatonin in concentrations of 10(-10) M and 10(-12) M (p < 0.01). TSH (20 mIU/ml), when applied alone, significantly decreased the mean volume, intersection area and perimeter of thyrocyte nuclei (p < 0.01). TSH generally did not counteract the melatonin-induced decrease of thyrocyte nuclei except for 10(-10) M melatonin concentration when TSH increased the mean nuclear perimeter (p < 0.01) and slightly enlarged the mean volume (p < 0.05), compared with Mel alone. The results suggest an inhibitory influence of melatonin on thyroid follicular cell activity, and are in compliance with earlier observations.