Does persistence of high thyroid levels (hyperthyroidism) require continuation of Methimazole (MMI) during pregnancy?

Management of Persistent Hyperthyroidism During Pregnancy

Yes, persistence of high thyroid levels (hyperthyroidism) requires continuation of Methimazole (MMI) during pregnancy, with the goal of maintaining free T4 or FTI in the high-normal range using the lowest possible dosage to minimize risks to both mother and fetus. 1, 2

Medication Selection Based on Trimester

• Propylthiouracil (PTU) is preferred during the first trimester due to lower risk of congenital malformations compared to MMI 2, 3
• MMI is preferred for second and third trimesters due to lower risk of hepatotoxicity 4
• If a patient is already on MMI when pregnancy is diagnosed, switching to PTU for the first trimester is recommended 3, 5

Monitoring and Dosage Adjustment

• Monitor free T4 or FTI every 2-4 weeks to guide appropriate dosage adjustments 1, 2
• Check TSH level every trimester to ensure stable thyroid function 1, 2
• The goal is to use the lowest possible thioamide dosage that maintains thyroid function in the high-normal range 1, 6
• Beta-blockers (e.g., propranolol) can be temporarily used to manage symptoms until thioamide therapy reduces thyroid hormone levels 1, 2

Risks of Inadequate Treatment

• Untreated or inadequately treated hyperthyroidism during pregnancy increases risks of:
  • Preeclampsia 1
  • Preterm delivery 6
  • Heart failure 2
  • Spontaneous abortion 4
  • Low birth weight in neonates 6
  • Fetal or neonatal thyroid dysfunction 1, 2

Medication Risks and Precautions

• MMI crosses the placental membrane and can cause:
  • Rare congenital defects when used in first trimester (aplasia cutis, craniofacial malformations, gastrointestinal malformations) 4, 7
  • Fetal goiter and cretinism if dosage is excessive 4
• Monitor for side effects of thioamides, particularly:
  • Agranulocytosis (presenting with sore throat and fever) 1, 4
  • Hepatitis, vasculitis, and thrombocytopenia 1, 4

Special Considerations

• In women with long-term treatment before conception, medication might be withdrawn in 40% of cases during first trimester if on low doses (<10 mg/day MMI) 6
• The newborn's physician should be informed about maternal hyperthyroidism due to the risk of neonatal thyroid dysfunction 1
• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy 1, 2
• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy 1

Postpartum Considerations

• Hyperthyroidism frequently relapses postpartum (in up to 83% of Graves' disease patients) 6
• MMI is the preferred treatment during lactation, as both MMI and PTU are present in breast milk but in amounts generally considered safe 4, 5

Remember that stopping antithyroid medication in the presence of persistent hyperthyroidism puts both mother and fetus at significant risk, making continued treatment essential despite the potential medication risks, which can be minimized through careful monitoring and appropriate dosage adjustment.