Thirty-seven pregnancies in 33 women with hyperthyroidism were divided into three groups: (1) those with no treatment during pregnancy; (2) those receiving antithyroid medication plus thyroid replacement therapy; and (3) those treated with antithyroid medication alone. There was no maternal mortality or notable morbidity. Of the viable infants, prematurity was substantially less in group 3.
We concluded that mothers treated with antithyroid drugs plus thyroid therapy appear to require higher doses of antithyroid drugs to reach comparable metabolic states, when compared to mothers receiving antithyroid medication alone. Inasmuch as thyroid crosses the placenta poorly while antithyroid drugs cross readily, the net effect for the fetus in combined therapy should be increased exposure to antithyroid drugs.
Mestman JH, Manning PR, Hodgman J. Hyperthyroidism and Pregnancy. Arch Intern Med. 1974;134(3):434-439. doi:10.1001/archinte.1974.00320210044006