Five groups of female rats which were exposed to thiouracil for varying periods around the time of birth were compared with a 6th group of untreated controls in motivational, metabolic, and hormonal test situations during adolescence and adulthood. The thiouracil-treated rats displayed reduced fearfulness in lever-touching and lever-pressing tasks in operant conditioning chambers and in their initial adaptation to activity-wheel and maze apparatuses. These rats also showed hyperactivity in asymptotic running-wheel performance, increased spontaneous recovery of extinguished lever-pressing, and elevated responding in lever-pressing for variable-interval food reinforcement. A supplemental study revealed significantly greater ad libitum food and water intake and oxygen consumption in male thiouracil-treated rats and elevated serum thyroxine levels in thiouracil-treated females. In general the results indicate that perinatal thyroid deficiency engenders a chronic hypermetabolic state in both sexes which may be associated with a persistent, mild hyperthyroid condition in the case of female rats.

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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 (T 4 ) 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 T 3 -treated rats displayed large maze learning deficits which were comparable in size to those produced by fairly severe thyroid deficiency in the perinatal period.

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