Influence of thyrotropin-releasing hormone on the secretion of thyrotropin in neonatal rats.

Two hours after the ip administration of TRH antiserum, no change in serum TSH concentration was observed in the rat from birth through day 5 of life. Under the same conditions, a significant reduction in serum TSH was observed in 7- to 14-day-old rats. Similarly, in neonatal hypothyroidism, TSH levels did not change in 1-day-old rats, whereas a significant decrease was observed on day 7. The administration of synthetic TRH during the neonatal period induced a significant increase of serum TSH in the newborn; however, TSH release by the pituitary gland increased progressively from days 3-10. Immunoreactive TRH was undetectable in the serum of newborn rats. Adult levels were reached when the rats were 10 days old. It is concluded that neonatal pituitary-thyroid function in the rat is not physiologically dependent upon TRH secretion, although synthetic TRH is able to stimulate the secretion of TSH at birth.