Although the placenta is only a limited barrier for the transfer of thyroxine (T4) to the fetus, we have recently demonstrated that maternal T4 does not suffice to prevent the effects of in utero hypothyroidism. The current study presents a convenient and minimally invasive animal model to study whether maternal-fetal transfer of T4 administered to the pregnant rat corrects the newborn's in utero hypothyroidism. In this model pregnant rats receive the goitrogen methimazole in their drinking water from d 10 of gestation until birth, either alone or together with L-T4. Circulating T4 levels in the dams and newborn pups are measured from blood spotted on filter paper by RIA. Using blood T4 levels as the measure, in the present study we found similar effects for orally and intraperitoneally administered T4. This rat model will allow future studies of whether maternal-fetal T4 transfer can correct the detrimental neurobehavioral effects of intrauterine hypothyroidism.

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