Neonatal hyperthyroidism: Maturational acceleration and learning deficit in triiodothyronine-stimulated rats

Rat pups injected on Postnatal Days 2-4 with subcutaneous doses of triiodothyronine (T3 ) up to 4 micrograms/g showed substantial accelerations in the maturation of swimming behavior, righting reflexes, and eye opening which were greater than the accelerations produced by neonatal thyroxine (T4 ) in a previous study. They also showed significantly higher activity in stabilimeter cages on Postnatal Day 13 but not an expected earlier peak than normal rats in the ontogeny-of-arousal functions obtained in stabilimeter testing. As adults, the T3 -treated rats displayed large maze learning deficits which were comparable in size to those produced by fairly severe thyroid deficiency in the perinatal period.