Pregnancy, Thyroid Disease

Key Features

• Relatively common in pregnancy

• Overt thyroid disease, both hypothyroidism and hyperthyroidism, are associated with adverse pregnancy outcomes

• Women should be screened with thyroid function tests when

  • There is a history of a thyroid disorder

  • Symptoms suggesting thyroid dysfunction are present

• Overt hypothyroidism

  • Defined by an elevated serum TSH level with a depressed free thyroxine (FT₄) level

  • The most common etiology is Hashimoto (autoimmune) thyroiditis

  • During pregnancy, several factors occur that affect maternal thyroid hormones

    • Rising estrogen levels increase thyroxine binding globulin (TBG) serum concentrations, reducing FT₄ levels

    • Placental deiodinase promotes the turnover of T₄

    • Supplemental iron and prenatal multivitamins containing iron can bind to oral T₄ and reduce its intestinal absorption

• Subclinical hypothyroidism

  • Defined as an increased serum TSH and a normal FT₄ level

  • Early observational studies suggest that cognitive function is impaired in offspring of women with untreated subclinical hypothyroidism

  • Although some studies have found associations with untoward pregnancy outcomes such as miscarriage, preterm birth, and preeclampsia, others have failed to confirm these findings

  • There is currently no evidence that identification and treatment of subclinical hypothyroidism will prevent any of these outcomes

• Overt hyperthyroidism

  • Defined as excessive production of thyroxine with a depressed (usually undetectable) serum TSH level

• 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

  • Associated with an increase in complications such as

    • Spontaneous abortion

    • Preterm birth

    • Preeclampsia

    • Placental abruption

    • Impaired neuropsychological development in the offspring

• Overt hyperthyroidism

  • Spontaneous abortion, preterm birth, preeclampsia, and maternal heart failure occur with increased frequency with untreated thyrotoxicosis

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

• 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 FT₄ level

• Overt hyperthyroidism

  • Excessive production of T4

  • Depressed (usually undetectable) serum TSH level

  • The TSH level generally stays suppressed even with adequate treatment

Treatment

• Overt hypothyroidism

  • For women who need levothyroxine, increase the dosages by approximately 20–30% as soon as pregnancy is confirmed because fetal CNS development partially depends on maternal T₄

  • Full replacement doses of 1.6 mcg/kg/day (about 100–150 mcg daily) should be given immediately to pregnant women with overt hypothyroidism or myxedema

  • By mid-pregnancy, women require an average of 47% increase in their levothyroxine dosage

  • Thyroid function tests can be ...