Late Effects of Thyroxine Injected into the Hypothalamus of the Neonatal Rat

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Abstract
Thyroxine (T4) in a systemically ineffective dose was implanted or injected into the hypothalamus of neonatal rats. Such treatment caused late and persistent alterations in pituitary-thyroidal and gonadal function similar to those seen in the ‘neo-T4 syndrome’ resulting from several doses of T4 given systemically to the neonate. This syndrome is characterized by a reduced body, pituitary, and thyroid weight, a reduced pituitary and serum TSH concentration, a reduced serum T4, diminished thyroidal function, and, in certain cases, delayed puberty, reduced ovarian weight, prolonged estrous cycles, and reduced fertility. Only T4 pellets in the arcuate ea of the hypothalamus caused a reduced pituitary TSH and reduced ovarian weight. We conclude that neonatal T4 may act directly to re-set certain developing hypothalamic centers regulating pituitary-thyroidal and gonadal function.

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