Acute Effects of Hypothalamic Ablation on Plasma Thyrotropin and Prolactin Concentrations in the Suckling Rat: Evidence That Early Postnatal Pituitary-Thyroid Regulation Is Independent of Hypothalamic Control*

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Abstract
Hypothalamic ablation was performed at various periods postnatally in animals previously administered propylthiouracil to raise plasma TSH concentrations. There was no significant change in plasma TSH up to 8 h after hypothalamic ablation in pups 1-4 days old, whereas hypophysectomy of such pups produced a 60% fall in plasma TSH within 4 h. By the 5th postnatal day, hypothalamic ablation produced a 30% fall in plasma TSH within 4 h (P < 0.05). By the 12th postnatal day and thereafter, the fall in plasma TSH after hypothalamic ablation was not significantly different from that seen in adults, except in 30-day-old rats in which there was a lesser effect of hypothalamic ablation on plasma TSH (P < 0.01 in comparison to 23-day-old and adult groups). The greatest effect of hypothalamic ablation on plasma TSH was in 45-day-old animals (P < 0.01 in comparison to adults). No significant change was produced in plasma PRL within 4 h postoperatively at any age. Our data indicate that regulation of TSH secretion in the rat is independent of hypothalamic control until after the 5th postnatal day and is fully developed by day 12. This corresponds temporally with the postnatal rise of plasma TSH, T4, and T3 and hypothalamic TRH to adult concentrations and indicates maturation of the hypothalamic regulation of TSH secretion.