What is the management of hyperthyroidism (excessive production of thyroid hormone) in pregnancy?

Management of Hyperthyroidism in Pregnancy

For hyperthyroidism in pregnancy, propylthiouracil is preferred in the first trimester, while methimazole is preferred in the second and third trimesters due to the risk of methimazole-associated teratogenicity in early pregnancy and propylthiouracil-associated hepatotoxicity later in pregnancy. 1, 2

Diagnosis and Monitoring

• Initial evaluation should include both TSH and Free T4 (FT4) or Free T4 Index (FTI) measurements 2
• Monitor thyroid function regularly:
  • As soon as pregnancy is confirmed
  • Every 4-6 weeks until TSH levels stabilize
  • At minimum during each trimester 2
• Goal: Maintain Free T4 in high-normal range using lowest possible thioamide dosage 2

Treatment Algorithm

First Trimester

1. Propylthiouracil (PTU) is the drug of choice 1, 2, 3
   • Reason: Methimazole is associated with potential teratogenicity during organogenesis 4
   • Use lowest effective dose to maintain FT4 in high-normal range 2

Second and Third Trimesters

1. Switch to Methimazole 1, 2, 3
   • Reason: PTU is associated with hepatotoxicity with prolonged use 1, 4
   • Monitor Free T4 every 2-4 weeks 2

Adjunctive Therapy

• Beta-blockers (e.g., propranolol) may be used for symptomatic relief 2
• Monitor prothrombin time during methimazole therapy, especially before surgical procedures 4

Special Considerations

Medication Dosing

• Aim for the lowest possible dose of thioamides to maintain FT4 in high-normal range 2
• In many pregnant women, thyroid dysfunction diminishes as pregnancy progresses, allowing for dose reduction 4
• In some cases, therapy can be discontinued several weeks or months before delivery 4

Gestational Transient Thyrotoxicosis

• Often associated with hyperemesis gravidarum 5
• Usually requires only supportive care and hydration 2
• No routine thyroid testing unless other signs of hyperthyroidism are present 2

Thyroid Storm

• Medical emergency requiring immediate intervention
• Treatment includes:
  • Thioamide
  • Saturated solution of potassium iodide or sodium iodide
  • Beta-blockers
  • Supportive care 2
• Often precipitated by surgery, infection, labor, or delivery 2

Monitoring for Adverse Effects

• Methimazole: Monitor for:

  • Hypoprothrombinemia and bleeding 4
  • Agranulocytosis (presents with sore throat and fever) 2
  • Hepatitis, vasculitis, and thrombocytopenia 2

• Drug Interactions:

  • Anticoagulants: Increased activity of oral anticoagulants 4
  • Beta-blockers: May require dose reduction when patient becomes euthyroid 4
  • Digitalis: Serum levels may increase when patient becomes euthyroid 4
  • Theophylline: Clearance may decrease when patient becomes euthyroid 4

Postpartum Considerations

• Methimazole is considered safe during breastfeeding 2, 4, 3
• Monitor for postpartum thyroiditis (PPT), which affects 5-10% of women within the first year after delivery 2
• Women with PPT need long-term follow-up as 20-40% develop permanent hypothyroidism 2

Risks of Untreated Hyperthyroidism

• Maternal complications: heart failure, spontaneous abortion, preterm birth, stillbirth 4, 6
• Fetal complications: hyperthyroidism, goiter, growth restriction, cognitive impairment 4, 5, 6

Important Caveats

• Radioactive iodine is absolutely contraindicated during pregnancy and breastfeeding 2
• Thyroidectomy should be limited to severe cases unresponsive to medical therapy 7
• Failing to differentiate between postpartum thyroiditis and Graves' disease can lead to unnecessary treatment 2
• Achieving euthyroidism before pregnancy is optimal for maternal and fetal outcomes 1