Perinatal hypothyroidism in rats: persistent motivational and metabolic effects.

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.