What is the recommended management of hyperthyroidism in a pregnant woman?

Management of Hyperthyroidism in Pregnancy

First-Line Pharmacologic Strategy

Use propylthiouracil (PTU) exclusively during the first trimester, then switch to methimazole for the second and third trimesters. 1, 2 This trimester-specific approach minimizes both congenital malformations (associated with methimazole in early pregnancy) and maternal hepatotoxicity (associated with prolonged PTU use). 1, 2

Diagnostic Confirmation

• Measure both TSH and free T4 (or free thyroxine index) in any pregnant woman with suspected hyperthyroidism. 3, 1
• Diagnosis requires elevated free T4/FTI with suppressed TSH in the absence of thyroid mass or nodular goiter. 3, 1
• Use trimester-specific TSH reference ranges—failure to do so leads to misdiagnosis and undertreatment. 3
• Look for clinical features: tremors, tachycardia, heat intolerance, hypertension, goiter, eyelid lag/retraction, and pretibial myxedema (specific to Graves' disease). 3
• Test for TSH-receptor antibodies in women with current or prior Graves' disease to assess risk of fetal thyrotoxicosis. 1

Treatment Goals and Monitoring

• Target free T4 (or FTI) in the high-normal range using the lowest effective thioamide dose—this maintains mild maternal hyperthyroidism and prevents fetal thyroid suppression. 1, 2
• Check free T4/FTI every 2–4 weeks to guide dose adjustments throughout pregnancy. 1, 2
• Once stable and euthyroid, measure TSH each trimester. 1, 2
• Do not aim for mid-normal or low-normal free T4 levels, as this increases the risk of fetal hypothyroidism from overtreatment. 2

Critical Safety Monitoring

Agranulocytosis

• Instruct patients to report fever or sore throat immediately. 1, 2, 4
• Obtain a complete blood count urgently and discontinue the thioamide immediately if agranulocytosis is confirmed. 1, 2

Hepatotoxicity

• Severe liver injury occurs primarily with PTU doses ≥300 mg/day but has been reported at doses as low as 50 mg/day. 2
• This risk underscores the importance of switching to methimazole after the first trimester. 2

Other Adverse Effects

• Monitor for vasculitis, thrombocytopenia, and hepatitis. 1, 2, 4

Symptomatic Management

• Provide short-acting beta-blockers (e.g., propranolol) temporarily to control tremor, tachycardia, and palpitations until thyroid hormone levels normalize. 1, 2
• Discontinue beta-blockers once biochemical control is achieved. 2

Maternal and Fetal Risks of Inadequate Treatment

• Untreated or inadequately controlled hyperthyroidism increases the risk of:
  • Severe preeclampsia 3, 1
  • Preterm delivery 3, 1
  • Maternal heart failure 3, 1
  • Miscarriage 3, 1
  • Low birth weight 3, 1
• Transplacental passage of TSH-receptor antibodies can cause fetal thyrotoxicosis and neonatal thyroid dysfunction. 3, 1
• Monitor fetal heart rate and growth in women with Graves' disease. 1, 2

Special Clinical Scenarios

Hyperemesis Gravidarum

• Biochemical hyperthyroidism with severe nausea/vomiting rarely requires antithyroid therapy unless overt clinical signs of hyperthyroidism are present. 1, 2
• Routine thyroid testing is not recommended in isolated hyperemesis. 1

Thyroid Storm

• Recognize as a medical emergency: fever, marked tachycardia, altered mental status, gastrointestinal symptoms, arrhythmia. 1, 2
• Treat immediately without waiting for laboratory confirmation using PTU or methimazole, inorganic iodide (potassium or sodium iodide), dexamethasone, phenobarbital, and supportive care. 1, 2
• Avoid delivery unless absolutely necessary, as maternal and fetal outcomes worsen during active thyroid storm. 1, 2

Surgical Management

• Reserve thyroidectomy for patients who fail thioamide therapy or develop severe drug intolerance (agranulocytosis, marked hepatotoxicity). 1, 2
• If surgery is required, schedule it in the second trimester. 1, 2

Absolute Contraindications

• Radioactive iodine (I-131) is strictly contraindicated throughout pregnancy because it ablates the fetal thyroid. 1, 2, 4
• Women must wait at least 4 months after I-131 therapy before initiating breastfeeding. 1, 2, 4

Postpartum Care

• Women treated with PTU or methimazole during pregnancy may breastfeed safely, as only minimal drug amounts enter breast milk. 1, 2, 4
• Notify the newborn's care team of maternal Graves' disease due to the risk of neonatal thyroid dysfunction. 1, 2
• Transient fetal or neonatal thyroid suppression from maternal thioamide therapy usually requires no specific treatment. 1, 2
• Monitor for postpartum thyroiditis in women with a history of thyroid dysfunction. 2

Key Pitfalls to Avoid

• Do not postpone treatment while awaiting repeat thyroid tests—fetal harm can precede maternal symptom onset. 1
• Failing to switch from PTU to methimazole after the first trimester increases the risk of maternal hepatotoxicity. 2
• Never use methimazole in the first trimester when PTU is available because of its higher teratogenic risk (choanal and esophageal atresia). 1, 2
• Do not target TSH >2.5 mIU/L in the first trimester, as even subclinical elevations are linked to adverse pregnancy outcomes. 1