What is the primary anti-thyroid medication and its side effects?

Primary Anti-Thyroid Medication and Side Effects

Methimazole (MMI) is the primary anti-thyroid medication for treating hyperthyroidism, preferred over propylthiouracil (PTU) due to its once-daily dosing, lower cost, and reduced risk of major toxicity. 1

Primary Anti-Thyroid Medications

Methimazole (First-Line)

• Mechanism of action: Inhibits thyroid hormone synthesis without inactivating existing thyroid hormones 2
• Dosing:
  • Initial dose: 15-30 mg daily (single dose)
  • Higher doses (30 mg/day) for severe hyperthyroidism
  • Lower doses (15 mg/day) for mild to moderate cases 3
• Advantages:
  • Once-daily dosing
  • More effective at normalizing thyroid function compared to PTU 3, 4
  • Lower risk of hepatotoxicity than PTU 1
  • More cost-effective 1

Propylthiouracil (Second-Line)

• Reserved for specific situations:
  • Thyroid storm
  • Pregnancy (first trimester)
  • Lactating women 1
  • Patients who experience adverse reactions to methimazole 5

Side Effects of Anti-Thyroid Medications

Common Side Effects

• Rash
• Pruritus (itching) 5
• Mild hepatotoxicity (more common with PTU) 3

Serious Adverse Effects

1. Agranulocytosis:

   • Presents with sore throat and fever
   • Requires immediate discontinuation of medication and complete blood count
   • Rare but potentially life-threatening 6

2. Hepatitis:

   • Higher risk with PTU than methimazole
   • Requires monitoring of liver function 6

3. Vasculitis 6

4. Thrombocytopenia 6

Medication Interactions

• Digitalis glycosides: Increased serum levels when hyperthyroid patients become euthyroid; may require reduced dosage 2
• Theophylline: Decreased clearance when hyperthyroid patients become euthyroid; may require reduced dosage 2

Special Considerations

Pregnancy

• Methimazole is classified as Pregnancy Category D
• Associated with potential fetal harm if used during first trimester
• May cause goiter and cretinism in the developing fetus
• PTU preferred during first trimester; consider switching to methimazole for second and third trimesters 2

Breastfeeding

• Methimazole is present in breast milk but generally considered safe
• Long-term studies have not demonstrated toxicity in nursing infants 2

Pediatric Use

• Methimazole is preferred over PTU in pediatric patients due to lower risk of severe liver injury 2

Monitoring During Treatment

• Monitor thyroid function (TSH, free T4) every 2-4 weeks initially
• Maintain free T4 or free T3 index in high-normal range using lowest possible dose
• Watch for symptoms of agranulocytosis (sore throat, fever)
• Adjust dosage based on thyroid function test results 6

Management of Side Effects

• For mild reactions like rash or itching, consider desensitization to methimazole under allergist supervision 5
• For serious reactions (agranulocytosis, hepatotoxicity), discontinue medication immediately and switch to alternative treatment 6

Methimazole remains the cornerstone of medical therapy for hyperthyroidism, with PTU reserved for specific clinical scenarios. Understanding the side effect profile and appropriate monitoring parameters is essential for safe and effective management of hyperthyroidism.