Maternal thyroid function: interpretation of thyroid function tests in pregnancy.

Thyroid status is frequently assessed during pregnancy, both to evaluate suspected thyroid abnormalities, and to monitor the status, of pre-existing thyroid disease. However, the production, circulation, and disposal of thyroid hormone are all altered in pregnancy. Interpretation of thyroid function studies in the pregnant patient must be performed based on an understanding of the normal physiologic changes at each stage of pregnancy. Examples of pregnancy-associated changes include, estrogen-stimulated increase in serum thyroxine binding globulin, chorionic gonadotropin stimulated T4 and T3 production from the thyroid gland, and accelerated degradation of thyroid hormone by the placenta. The serum-TSH alone is usually not adequate to assess thyroid status in pregnancy, and the various conditions that can effect this measurement are described. The pattern of thyroid studies in pregnancy is important in diagnosing thyroid disease, and it may indicate physiologic adaptations to optimize maternal thyroid status for fetal development.