Effects of neonatal hypo- and hyperthyroidism on pituitary growth hormone content in the rat

Thyroid hormones play an important role in growth and development. Therefore, we investigated the effects of neonatal hypo- and hyperthyroidism on pituitary GH content in the rat. In control rats, pituitary GH content increased from 4.16 +/- 0.34 at 2 days to 43.7 +/- 4.2 microgram/gland (mean +/- SE) at 15 days of age, with a t 1/2 of increment of 3.48 +/- 0.40 days. Between 18-60 days of age, pituitary GH content increased from 56.9 +/- 4.0 to 300 +/- 28 microgram/gland, with a t 1/2 of 18.2 +/- 1.5 days. The administration of T3 had no significant effect on the pituitary GH content of these animals. In neonatal hypothyroid rats, pituitary GH content was significantly lower than that of controls at 2 days of age (P < 0.01) and decreased from 8 days on, with a t 1/2 of 3.71 +/- 0.25 days. However, 24 h after the administration of T3 (100 microgram/100 g BW), pituitary GH content was significantly increased in these animals. Similarly, the administration of T3 (0.4 microgram/100 g BW) to 14-day-old hypothyroid rats restored the pituitary GH content to 70-80% of normal after 5 days of therapy. Conversely, hyperthyroidism induced in 14-day-old normal or hypothyroid rats resulted in a significant decrease in their pituitary GH contents after 5 days of treatment. Therefore, the present results indicate that during the neonatal period, thyroid hormones play a primary role in the control of GH accumulation in the pituitary. Furthermore, the lack of increase in pituitary GH content after the administration of T3 during development might suggest that the rate of formation of GH is already maximum during this period of life in the rat, or, alternatively, that the pituitary nuclear T3 receptors are near full saturation during development. Finally, a generally similar effect of T3 on pituitary GH response was observed in the neonatal rat as well as in the adult animal.