Persistent abnormalities in pituitary function following neonatal thyrotoxicosis in the rat

Studies of pituitary and thyroid function were conducted in adult rats with the syndrome of neonatal thyrotoxicosis (NT) produced by the administration of large doses of L-thyroxine during the first week of postnatal life. As compared to normal controls,adult NT rats displayed decreased thyroid I31I uptake, an increase in the proportion ofthyroid organic 131I present as monoiodotyrosine and a decreased proportion of the iodothyronines,thyroxine (T4)and triiodothyronine, findings consis-)tent with those seen in association with deficiency of TSH. In addition,serum T4 concentrations were consistently lower in adult NT than in control rats. Adult NT rats displayed marked decreases in TSH secretory reserve, as indicated by lower values of basal serum TSH concentration following thyroidec-tomy and far lower increments in serum TSH concentration following standard doses of TRH iv, even when NT and control rats had all been thyroidectomized and maintained for long periods on replacement doses of L-thyroxine. Abn...