A 29-year-old G1 at 9 weeks’ gestation presents to your office for a new OB visit. She reports a history of well-controlled hypothyroidism. She takes 88 mcg of levothyroxine daily.
How do you expect her thyroid laboratory values to change during pregnancy?A. The thyroid-stimulating hormone (TSH) and free T4 will not change during pregnancy
There are considerable changes in maternal thyroid function during pregnancy. Maternal total or bound thyroid hormone levels increase with serum concentration of thyroid-binding globulin. TSH decreases in early pregnancy because of weak stimulation of its receptors by human chorionic gonadotropin (hCG) during the first trimester After the first trimester, TSH levels return to baseline values and progressively increase in the third trimester related to placental growth and production of placental deiodinase. Free T4 remains stable during pregnancy. A high TSH and low free T4 are characteristic of overt hypothyroidism. These physiologic changes should be considered when interpreting thyroid function test results during pregnancy.