Pregnancy, Thyroid Disease

Key Features

• Overt hypothyroidism

  • Associated with an increase in complications such as preterm birth, preeclampsia, placental abruption, and impaired neuropsychological development in the offspring

  • The most common etiology is Hashimoto (autoimmune) thyroiditis

• Subclinical hypothyroidism

  • Defined as an increased TSH and a normal FT₄ level

  • American College of Obstetricians and Gynecologists and the American Association of Clinical Endocrinologists recommend against screening for or treating subclinical hypothyroidism

• Overt hyperthyroidism

  • Associated with spontaneous abortion, preterm birth, preeclampsia, and maternal heart failure if left untreated

  • Thyroid storm, although rare, can be a life-threatening complication

• Transient autoimmune thyroiditis can occur in the postpartum period and is evident within the first year after delivery

Clinical Findings

• Overt hypothyroidism: Symptoms mimic those of normal pregnancy

• Overt hyperthyroidism

  • Fetal hypothyroidism or hyperthyroidism are uncommon but can occur with maternal Graves disease, which is the most common cause of hyperthyroidism in pregnancy

• Transient autoimmune thyroiditis

  • The first phase, occurring up to 4 months postpartum, is a hyperthyroid state

  • Over the next few months, there is a transition to a hypothyroid state, which may require treatment with levothyroxine

Diagnosis

• Screening with thyroid function tests

  • Indicated for women who have a history of a thyroid disorder or symptoms that suggest thyroid dysfunction

  • No proven benefit for asymptomatic pregnant women and not currently recommended

• Overt hypothyroidism

  • Elevated serum TSH level

  • Depressed serum FT₄ level

Treatment

• Overt hypothyroidism

  • Levothyroxine, 75–100 mcg orally daily

  • Thyroid function tests can be repeated at 4–6 weeks and the dose adjusted as necessary with the goal of normalizing the TSH level

  • An increase in the dose of levothyroxine may be required in the second and third trimesters

• Overt hyperthyroidism

  • Propylthiouracil or methimazole

  • A beta-blocker can be used to manage palpitations or tremors

  • Radioiodine ablation must be avoided because it may destroy the fetal thyroid as well

• Transient autoimmune thyroiditis: Spontaneous resolution to a euthyroid state within the first year is the expected course; however, some women remain hypothyroid longer