Effect of maternal hypothyroidism and triiodothyronine on the fetus and newborn in rats.

Of 36 adult female rats studied 19 were radiothyroidectomized; 3 weeks later all rats were bred. Radiothyroidectomized rats were divided into Groups I-III. Group I was untreated. Maternal weight gain was not noted until day 9 and was steady thereafter. Group III rats were treated with triiodothyronine up to the first day of pregnancy, but were hypothyroid during gestation (elevated TSH and low T3). Group II rats were treated with T3 up to the day of delivery. The pregnancy, its outcome, and TSH and T3 levels were similar to the normal controls (Group VI). Pregnancy, its outcome, and TSH and T3 levels were not different from controls when T3 was given to normal rats up to day 1 of pregnancy (Group IV). T3 concentrations were elevated in the normal rats which were given T3 continuously up to the day of delivery (Group V). However, the animals did not appear to be thyrotoxic and TSH was not suppressed. The size of the litters of Group I mothers was significantly less than in other groups and this difference may be attributed to an increased tendency to resorb implanted fetuses in untreated hypothyroid mothers. However, activity, day of eye opening weight gain, and TSH and T3 levels of the pups of all the groups were similar up to age 21 days. The data indicate the independence of the fetal pituitary-thyroid axis from maternal thyroidal status in rats.