Effect of rendering male rats hypothyroid at different stages of development on adenohypophysial hormone release

The hypothalamic-pituitary axis was studied in male rats rendered hypothyroid at different stages in their development. Rats were injected with 131I or H2O (controls) within 12 h after birth, at 5 d or 15 d of life. Their thyroid stimulating hormone (TSH) responses to thyrotropin releasing hormone (TRH), their prolactin (PRL) responses to Haloperidol (H) and TRH and their luteinizing hormone (LH) responses to luteinizing hormone-releasing hormone (LH-RH) were then studied at 30 and 75 d of life. Basal and TRH-stimulated TSH release was markedly greater in the rats radiothyroidectomized within 12 h of birth than for the other 3 groups, probably reflecting greater thyroid damage at a critical time in development. The basal serum PRL and the PRL responses to TRH and H were markedly less in the rats rendered hypothyroid within 12 h after birth than in the other 2 groups of rats. Basal and LH-RH induced LH secretion were greater at 30 d in the rats rendered hypothyroid at 5 and 15 d than in the other 2 groups. Although basal LH levels were higher at 75 d in the rats rendered hypothyroid at 5 and 15 d, their LH responses to LH-RH were similar to the controls and rats made hypothyroid within 12 h. The greatest alterations in TSH and PRL secretion occur when animals are rendered hypothyroid very early in neonatal life. The hypergonadotropic state of the animals rendered hypothyroid at 5 and 15 d is reminescent of juvenile hypothyroidism in man.