Developmental changes in brain TRH and in plasma and pituitary TSH and prolactin levels in the rat.

TRH in the hypothalamus and the rest of the brain, as well as TSH and prolactin in the pituitary gland and the plasma have been determined by radioimmunoassay in rats varying in age (10- to 22-day-old fetuses and 1- to 60-day-old rats). TRH is first detected on the 16th day of gestation and its maximum increase occurs during the first 3 weeks of life both in the hypothalamus and the rest of the brain. The evolution patters of TSH and prolactin in the plasma and the pituitary gland are discussed in relation to TRH levels in the brain and in the hypothalamus.