What is the recommended treatment for hyperthyroidism in pregnant women?

Treatment of Hyperthyroidism in Pregnant Women

For hyperthyroidism in pregnancy, propylthiouracil (PTU) should be used in the first trimester, followed by switching to methimazole (MMI) in the second and third trimesters to minimize risks of teratogenicity and hepatotoxicity respectively. 1

Medication Selection Algorithm

1. First Trimester (0-13 weeks)

   • Use propylthiouracil (PTU) as first-line therapy
   • Rationale: Lower risk of congenital anomalies compared to methimazole 1, 2
   • PTU may be the treatment of choice when an antithyroid drug is indicated during or just prior to the first trimester 3

2. Second and Third Trimesters (>13 weeks)

   • Switch from PTU to methimazole (MMI)
   • Rationale: Avoid PTU-associated hepatotoxicity which can be severe 1, 3

3. Dosing and Monitoring

   • Goal: Maintain Free T4 in the high-normal range using lowest possible thioamide dosage 1
   • Monitor Free T4 every 2-4 weeks during treatment adjustment 1
   • Monitor prothrombin time during therapy, especially before surgical procedures (for MMI) 4

Special Considerations

Pre-pregnancy Planning

• Achieving euthyroidism before pregnancy is optimal for maternal and fetal outcomes 1
• Consider definitive therapy (surgery or radioactive iodine) before pregnancy when appropriate to avoid medication during pregnancy 5
• Radioactive iodine is contraindicated during pregnancy 1

Risks of Untreated Hyperthyroidism

• Maternal complications: heart failure, spontaneous abortion, preterm birth, stillbirth 4
• Fetal complications: hyperthyroidism, growth restriction, fetal death 6

Medication-Specific Risks

• Methimazole (MMI):

  • Higher risk of congenital anomalies when used in first trimester 2
  • Pattern of anomalies includes choanal atresia, aplasia cutis congenita, facial, heart, and gastrointestinal defects 7
  • Crosses placenta and can induce goiter and cretinism in developing fetus 4

• Propylthiouracil (PTU):

  • Risk of severe hepatotoxicity, including liver failure requiring transplantation 3
  • Lower risk of congenital anomalies compared to MMI 2, 8

Dose Adjustments During Pregnancy

• Thyroid dysfunction often diminishes as pregnancy progresses
• Reduction of dosage may be possible in later pregnancy 4
• In some cases, therapy can be discontinued several weeks or months before delivery 4

Breastfeeding Considerations

• Both MMI and PTU are considered safe during breastfeeding 1, 4
• MMI is the preferred medication during lactation 6
• Long-term studies have not demonstrated toxicity in infants nursed by mothers taking MMI 4

Emergency Situations

• For thyroid storm: immediate intervention with thioamide, potassium/sodium iodide, beta-blockers, and supportive care 1
• Monitor for rare but serious side effects: agranulocytosis, hepatitis, vasculitis, thrombocytopenia 1

Drug Interactions

• Beta-blockers: Hyperthyroidism increases clearance; dose reduction needed when patient becomes euthyroid 4
• Anticoagulants: Activity may increase; monitor PT/INR 4
• Digitalis: Serum levels may increase when patient becomes euthyroid 4
• Theophylline: Clearance may decrease when patient becomes euthyroid 4

Remember that the goal of treatment is to control maternal hyperthyroidism with the lowest effective dose of antithyroid medication to minimize fetal exposure while maintaining maternal euthyroidism.