Thyroid function during pregnancy

Background: This Case Conference reviews the normal changes in thyroid acti vity that occur during pregnancy and the proper use of laboratory tests for the diagnosis of thyroid dysfunction in the pregnant patient. Case: A woman in the 18th week of pregnancy presented with tachycardia, inc reased blood pressure, severe vomiting, increased total and free thyroid ho rmone concentrations, a thyroid-stimulating hormone (TSH) concentration wit hin the reference interval, and an increased human chorionic gonadotropin ( hCG) beta-subunit concentration. Issues: During pregnancy, normal thyroid activity undergoes significant cha nges, including a two- to three-fold increase in thyroxine-binding globulin concentrations, a 30-100% increase in total triiodothyronine and thyroxine concentrations, increased serum thyroglobulin, and increased renal iodide clearance. Furthermore, hCG has mild thyroid stimulating activity. Pregnanc y produces an overall increase in thyroid activity, which allows the health y individual to remain in a net euthyroid state. However, both hyper- and h ypothyroidism can occur in pregnant patients. In addition, two pregnancy-sp ecific conditions, hyperemesis gravidarum and gestational trophoblastic dis ease, can lead to clinical hyperthyroidism. The normal changes in thyroid a ctivity and the association of pregnancy with conditions that can cause hyp erthyroidism necessitates careful interpretation of thyroid function tests during pregnancy. Conclusion: Assessment of thyroid function during pregnancy should be done with a careful clinical evaluation of the patient's symptoms as well as mea surement of TSH and free, not total, thyroid hormones. Measurement of thyro id autoantibodies may also be useful in selected cases to detect maternal G raves disease or Hashimoto thyroiditis and to assess risk of fetal or neona tal consequences of maternal thyroid dysfunction. (C) 1999 American Associa tion for Clinical Chemistry.