Thyroxine requirement during pregnancy for replacement therapy of hypothyroidism.

We examined the dose requirements of thyroxine (T4) and desiccated thyroid during eight pregnancies of six women who had undergone total thyroidectomy for thyroid carcinoma. In five pregnancies from four patients treated with T4, serum free T4, which was measured by a newly developed radioimmunoassay, decreased during pregnancy but increased above the normal range after delivery. Consistent with these changes in free T4, serum TSH (measured by a highly sensitive immunoradiometric assay) increased during pregnancy but returned to an undetectable level after delivery, with one exception. The serum triiodothyronine (T3)-to-T4 ratio, which is related to peripheral conversion of T4 to T3, was lower in patients treated with T4 than in normal controls, regardless of pregnancy. The ratio decreased further during pregnancy, and so relative deficiency of T3 during pregnancy was suspected, especially in T4-treated patients. On the other hand, in two pregnancies from two other patients treated with desiccated thyroid at a dose most equivalent to that of T4, serum free T4 decreased to a low-normal value during gestation but returned to the normal range after delivery, whereas serum TSH scarcely changed during pregnancy. These findings indicate that replacement therapy for pregnant patients with hypothyroidism after total thyroidectomy should include an increased dose of T4; in contrast, the dose of desiccated thyroid need not be changed.