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Delayed vaginal opening in the rat following pharmacologic doses of T4 administered during the neonatal period.

Abstract In view of evidence that there is a "critical" neonatal period of neural development controlling pituitary-gonadal function, the effects of L-thyroxin (T 4 ) administered to rats during the first 5 days of life were investigated with regard to the onset of puberty and pituitary-thyroidal function. After treatment the rats were found to have reduced body growth, reduced pituitary thyrotropin (TSH) levels, and slight hypothyroidism as demonstrated by depressed thyroid weights, reduced serum protein-bound iodine (PBI), and free T 4 levels. Such animals also exhibited retarded vaginal opening and first estrus. It is thought that these changes may reflect a persistently altered hypothalamic mechanism induced by exposure to excessive thyroid hormone during a "critical" period of development.