What is the best management approach for a pregnant woman with hyperthyroidism (excessive production of thyroid hormones)?

Management of Hyperthyroidism in Pregnancy

Propylthiouracil (PTU) should be used as first-line treatment during the first trimester, then switched to methimazole for the second and third trimesters to minimize both teratogenic risk and maternal hepatotoxicity. 1

Medication Selection by Trimester

First Trimester (Weeks 0-13)

• Use PTU exclusively during organogenesis (weeks 6-10) because methimazole is associated with congenital malformations including aplasia cutis and other birth defects 1, 2, 3
• PTU crosses the placenta minimally (only 0.025% into breast milk) compared to methimazole 1
• If a patient is already on methimazole when pregnancy is discovered, switch immediately to PTU 4

Second and Third Trimesters (Weeks 14-40)

• Switch from PTU to methimazole (up to 30 mg/day is safe) to reduce maternal risk of hepatotoxicity 1, 4
• This trimester-specific approach balances fetal teratogenic risk against maternal liver toxicity 4, 2

Treatment Goals and Monitoring

Target Thyroid Levels

• Maintain maternal free T4 (FT4) or free thyroxine index (FTI) in the high-normal range or just above normal using the lowest effective thioamide dose 1, 5
• Avoid overtreatment, as this increases risk of fetal hypothyroidism and goiter 6, 7

Monitoring Frequency

• Check FT4 or FTI every 2-4 weeks during active treatment until stable 1
• Once TSH is stable, monitor every 4 weeks 1
• Use free thyroid hormone levels (not total T4/T3) because total levels are elevated in normal pregnancy due to increased thyroxine-binding globulin 6

Drug Withdrawal Considerations

• In women on long-term ATD treatment before conception with well-controlled disease, consider withdrawing medication in the first trimester if on low doses (MMI <10 mg/day) 7
• Approximately 40% of such patients can successfully discontinue therapy, though 14% will relapse 7
• Many patients can discontinue therapy several weeks to months before delivery as thyroid dysfunction often diminishes with pregnancy progression 2

Critical Safety Monitoring

Immediate Reporting Requirements

Instruct patients to immediately report the following and obtain complete blood count if suspected 1:

• Sore throat and fever (agranulocytosis warning signs)
• New rash, hematuria, decreased urine output, dyspnea, or hemoptysis (vasculitis indicators) 1

Additional Monitoring

• Monitor for hepatitis and thrombocytopenia 1
• Check prothrombin time, especially before surgical procedures, as methimazole may cause hypoprothrombinemia 2

Thyroid Storm Management

This medical emergency affects <1% of pregnant women with hyperthyroidism but requires aggressive intervention 5:

Pharmacologic Protocol

Administer the following drug series 1:

• Propylthiouracil or methimazole
• Saturated solution of potassium iodide or sodium iodide
• Dexamethasone
• Phenobarbital

Supportive Care

• Provide oxygen, antipyretics, and appropriate hemodynamic monitoring 1
• Avoid delivery during thyroid storm unless absolutely necessary 1
• Evaluate fetal status with ultrasound, nonstress testing, or biophysical profile depending on gestational age 1

Alternative Treatment Options

Surgical Thyroidectomy

Reserve for specific indications 1:

• Failure of medical therapy
• Large compressive goiters
• Strong patient preference for surgery
• Perform during the second trimester when safest 1

Contraindicated Treatments

• Radioactive iodine is absolutely contraindicated during pregnancy and lactation 1
• Women must not breastfeed for 4 months after I-131 treatment 1

Adjunctive Therapy

Beta-Blockers

• Use propranolol or other beta-adrenergic blockers for symptomatic relief while awaiting definitive treatment 5
• Note that beta-blocker dosing may need reduction as the patient becomes euthyroid due to decreased clearance 2

Consequences of Untreated Disease

Inadequately treated hyperthyroidism significantly increases maternal and fetal risks 5, 6:

• Severe preeclampsia
• Preterm delivery and stillbirth
• Maternal heart failure
• Miscarriage
• Low birth weight
• Fetal thyrotoxicosis (in women with history of Graves' disease due to transplacental antibody passage) 5

Fetal and Neonatal Considerations

Fetal Thyroid Development

• By 20 weeks' gestational age, the fetal thyroid is fully responsive to both thyroid-stimulating immunoglobulins and antithyroid drugs 6
• All antithyroid drugs cross the placenta and can induce fetal goiter and hypothyroidism if maternal dosing is excessive 2, 6

Neonatal Monitoring

• Consider neonatal immune-mediated hyperthyroidism or hypothyroidism in women with Graves' disease history due to transplacental antibody passage 5
• Neonatal hyperthyroidism occurs in approximately 3.3% of live births to mothers with Graves' disease 7

Postpartum Management

• Evaluate thyroid function 6 weeks after delivery to detect postpartum thyroiditis or Graves' disease recurrence 1
• Hyperthyroidism relapses postpartum in 83% of Graves' disease patients (median 3 months postpartum) 7
• Both propylthiouracil and methimazole are compatible with breastfeeding 5, 2

Screening Recommendations

Screen pregnant women with 1:

• Symptoms of thyroid disease
• History of thyroid disease
• Thyroid nodules or goiter
• Universal screening is not currently recommended 1

Common Pitfalls to Avoid

• Do not use methimazole during first trimester organogenesis (weeks 6-10) due to teratogenic risk 1, 3
• Do not continue PTU throughout entire pregnancy due to maternal hepatotoxicity risk 4, 2
• Do not aim for normal TSH levels—target high-normal or slightly elevated FT4 instead to avoid fetal hypothyroidism 1, 7
• Do not use total T4/T3 levels for monitoring—these are physiologically elevated in normal pregnancy 6
• Do not overlook the need for more frequent monitoring in women diagnosed during pregnancy versus those with long-term pre-conception control 7