To investigate whether maternal hypothyroidism before the onset of fetal thyroid function influences mental development of the offspring.
We examined IQs in children in whom the mothers had been hypothyroid during early pregnancy (group 1). The IQs were compared with those of siblings who were not exposed to maternal hypothyroidism during gestation (group 2).
Group 1 consisted of eight children. Mothers were examined for thyroid status during the fifth to 10th gestational weeks and were found to have distinctly low thyroxine levels and high thyrotropin levels; the levels became normal after thyroxine supplementation by 13 to 28 weeks of gestation. Seven of the eight children had nine siblings who had not been exposed to maternal hypothyroidism during gestation (group 2). Ages at examination were 4 to 10 years in group 1 and 4 to 15 years in group 2.
All children in group 1 showed normal IQs. There was no significant difference in the mean IQ between the children in group 1 who had siblings (112±11) and their siblings in group 2 (106±8). Even the subject whose mother had had the lowest thyroxine level (free thyroxine, 2.3 pmol/L) had an IQ similar to that of his sibling.
These data provide evidence against the presence of adverse effects of maternal hypothyroidism during early pregnancy on the subsequent mental development of the offspring.(Arch Intern Med. 1994;154:785-787)