Maternal hypothyroidism: recognition and management.

Women with compensated early thyroid failure, or those from areas of reduced iodine intake, may first be found to be hypothyroid during pregnancy. In women with previously diagnosed hypothyroidism already on thyroxine (T4) replacement therapy, pregnancy is often associated with an increased dose requirement. The mechanism producing this increased requirement is not known, but it is likely to be the result of a number of factors that may differ depending on the stage of pregnancy. An increased T4 dose requirement is typically seen by the first trimester, can continue to increase throughout pregnancy, and reverts to the prepregnancy dose requirement after delivery. The magnitude of the increased T4 requirement is related to the etiology of hypothyroidism. Monitoring thyroid status and adjusting the T4 dose during pregnancy is a challenge due to changes in T4 metabolism throughout pregnancy.