In developing female rats, pituitary content and concentration of TSH and prolactin measured twice daily (1000 and 1600 h) were low during the first two weeks of life and increased markedly between day 15 and 30. AM-PM variations in pituitary levels of both hormones were apparent, particularly during this latter phase of development. Plasma TSH levels increased between days 5 and 12, showing peak values at this age. After day 15, levels declined gradually to reach a nadir shortly before puberty. At puberty, plasma TSH remained low, showing only minor fluctuations. No consistent AM-PM differences in plasma TSH were observed at any age studied. Plasma prolactin was low between day 5 and 15, increasing thereafter. Starting at day 10, AM-PM fluctuations in plasma levels were detected, titers being higher in the afternoon than in the mornings. It has already been reported (Endocrinology 98: 630, 1976) that during puberty this pattern becomes more evident, peak values being reached in the afternoon of the first proestrus. Hypothalamic TRH content increased between day 5 and 15, reaching a maximum at this age and declining thereafter to adult values. No qualitative changes in pituitary TSH during development were observed, as determined by exclusion chromatography. The existence of divergent patterns of plasma TSH and prolactin during female sexual maturation and the fact that hypothalamic TRH content could be more easily correlated with circulating TSH titers than with prolactin levels suggest that the mechanism(s) that stimulates pituitary release of these two hormones during sexual development is different.
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