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For further information, see CMDT Part 19-18: Thyroid Disease in Pregnancy

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 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 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 serum FT4 level

  • Overt hyperthyroidism

    • Elevated serum T4 level

    • 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 repeated at ...

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