Background The present study compared behavioral and molecular indicators of hippocampal function in l-thyroxine treated rats to determine whether thyroid hormone excessiveness produces relatively stable lifelong changes. Methods Hyperthyroidism was induced in rats by daily injections of l-thyroxine (0.2 mg/kg) to their dams for lactation period (MOH: maternal-onset hyperthyroidism) or to the rats itself during the young adult period (AOH: adult-onset hyperthyroidism; between the day 39-60). Spatial learning was assessed in the Morris Water Maze (MWM). Levels of type 2 and type 3 deiodinases, Erk1/2, JNK and P38(MAPK) were assessed via western blotting in the hippocampus of trained rats. Measurements were all done in rats aged 60-66 days. Results In MWM, maternally treated rats with l-thyroxine swam more away from the hidden platform, with showing more anxiety-like behavior, as compared to the rats treated or no treated with l-thyroxine in young adulthood. In spite of impaired acquisition, MOH group was not significantly different from the other groups in probe trial. In Western blot of the hippocampus, a decreased the expression of P38MAPK was found in rats treated with l-thyroxine in young adulthood period. However, maternal treatment with l-thyroxine resulted in an increased expression of Type 2 deiodinase and a tendency toward decreased expression of total and phosphorylated ERK1/2. No detectable band for type 3 deiodinase, p-JNK and p-P38(MAPK) was observed in all three groups. Conclusion These results suggest that perinatal excessiveness of thyroid hormone has longstanding effects on hippocampal function and may account for memory problems experienced by adolescents with lactational hyperthyroidism.