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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 β-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

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