Management of thyroxine therapy during pregnancy

To review the management of thyroxine (T4) therapy in pregnant patients with hypothyroidism.

The results of pertinent published studies are summarized, and practical recommendations are presented.

The conditions for which T4 therapy is administered during pregnancy are the same as those in nonpregnant patients: hypothyroidism, thyrotropin or thyroid-stimulating hormone (TSH) control after surgical treatment of thyroid cancer, and, in selected patients, suppression treatment for postsurgical thyroid remnants, thyroid nodules, or goiter. Untreated hypothyroidism during pregnancy can potentially cause adverse effects in both mother and fetus. Up to 75% of T4-treated women with hypothyroidism require higher doses of T4 during pregnancy than before or after conception, to maintain serum TSH levels in the normal range. Otherwise, in a substantial percentage of these women, subnormal serum free T4 levels, TSH elevations >20 microIU/L, or both will develop. The mean T4 dose needed to correct hypothyroidism during pregnancy is about 150 microg/day, but individual dose requirements vary widely.

The increment in T4 dose needed to normalize an increased TSH level in women taking T4 can be estimated from the serum TSH concentration during pregnancy. Increased TSH levels can appear as early as 4 to 8 weeks of gestation or as late as the third trimester. Although the optimal schedule is uncertain, assessing the TSH once each trimester seems reasonable. After pregnancy, the T4 dose should be reduced to the preconception level, and postpartum reassessment should be done at 6 to 12 weeks.