AbstractBackground: This Case Conference reviews the normal changes in thyroid activity 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, increased blood pressure, severe vomiting, increased total and free thyroid hormone concentrations, a thyroid-stimulating hormone (TSH) concentration within the reference interval, and an increased human chorionic gonadotropin (hCG) β-subunit concentration.Issues: During pregnancy, normal thyroid activity undergoes significant changes, including a two- to threefold 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. Pregnancy produces an overall increase in thyroid activity, which allows the healthy individual to remain in a net euthyroid state. However, both hyper- and hypothyroidism can occur in pregnant patients. In addition, two pregnancy-specific conditions, hyperemesis gravidarum and gestational trophoblastic disease, can lead to clinical hyperthyroidism. The normal changes in thyroid activity and the association of pregnancy with conditions that can cause hyperthyroidism 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 measurement of TSH and free, not total, thyroid hormones. Measurement of thyroid autoantibodies may also be useful in selected cases to detect maternal Graves disease or Hashimoto thyroiditis and to assess risk of fetal or neonatal consequences of maternal thyroid dysfunction.