Thyroid Disease

Several special considerations of thyroid dysfunction during pregnancy should be kept in mind. First, signs and symptoms of disordered thyroid function may be mimicked by pregnancy itself. Second, increases in thyroid-binding proteins during pregnancy alter standard thyroid function tests. Third, placental transfer of most thyroid hormones is minimal. Finally, most antithyroid drugs freely cross the placenta and are potentially active in the fetal thyroid.

Profound hypothyroidism is often associated with impaired fertility and is uncommon during pregnancy. On the other hand, mild hypothyroidism is a frequent concomitant illness during pregnancy. Therapy consists of full thyroid hormone replacement. When replacement is adequate, thyroid-stimulating hormone levels return to normal, but this may take up to 8 weeks for maximum effect. There is no evidence to suggest that replacement dosages of thyroid medication suppress fetal thyroid function.

Graves' disease is the major cause of thyrotoxicosis in women of childbearing age and thus in pregnancy. The clinical course of Graves' disease is known to vary during gestation. Thyrotoxicosis often improves during pregnancy and worsens postpartum.

Since radioactive iodide therapy is contraindicated during pregnancy, therapeutic options include antithyroid medications and surgery. Two important points require emphasis before specific treatment of hyperthyroidism is considered. First, mild hyperthyroidism appears to be well tolerated during pregnancy. Second, antithyroid drugs freely cross the placenta and are active against the fetal thyroid.