Purpose of review: Examine recent studies on the assessment of thyroid status in pregnancy, approach to thyroid testing, the spectrum of hypothyroidism in pregnancy, and strategies for thyroid replacement in women with known hypothyroidism.
Recent findings: Trimester-specific references range for thyroid-stimulating hormone (TSH) and free thyroxine in pregnancy must take into account iodine and thyroid autoantibody status, race, BMI, as well as other factors. Thyroid testing of only those pregnant women at increased risk for thyroid disease, case finding, will miss 30-80% of women with thyroid disease. Subclinical hypothyroidism is associated with an increasing number of adverse effects including infertility, miscarriage, preterm delivery, and breech presentation at birth. Many pregnant women with known hypothyroidism have an out-of-range TSH at the time of confirmed pregnancy. A variety of strategies are effective at keeping serum TSH normal during pregnancy including preconception increase in thyroxine, increase in thyroxine dose at the time pregnancy is confirmed, or making adjustments based on serum TSH monitoring.
Summary: Evaluation of thyroid status in pregnancy requires an understanding of pregnancy-associated changes in thyroid function tests and how they vary by trimester. The spectrum of hypothyroidism in pregnancy includes isolated thyroid peroxidase antibody positivity, isolated hypothyroxinemia, subclinical and overt hypothyroidism. These patterns, in some situations, may be related to iodine status, selenium status, or underlying thyroid disease. There are a variety of approaches to management of thyroxine replacement in known hypothyroid women at the time of pregnancy that are all effective at maintaining a normal range during pregnancy.