Can methimazole (antithyroid medication) cause headaches?

Methimazole and Headaches

Yes, methimazole can cause headaches as a side effect, though it is not among the most commonly reported adverse reactions. 1

Evidence for Methimazole-Associated Headaches

The FDA drug label for methimazole specifically lists headache as a symptom that patients should report immediately to their healthcare provider 1. This is particularly important because headache can sometimes be part of a constellation of symptoms indicating more serious adverse reactions.

A recent meta-analysis examining the effects of methimazole (combined with propranolol) found that headache was among the adverse effects monitored, though there was no significant difference in headache incidence between patients taking methimazole alone versus methimazole with propranolol 2.

Common Side Effects of Methimazole

Methimazole is associated with several side effects, with headache being less common than:

• Skin eruptions and pruritus
• Gastrointestinal symptoms
• Fever
• General malaise
• Sore throat (which may indicate agranulocytosis)

Clinical Significance and Management

When a patient on methimazole reports headaches:

1. Evaluate for serious adverse reactions: Headache may be part of a more serious reaction like vasculitis 1

   • Look for accompanying symptoms such as:
     • Rash
     • Hematuria or decreased urine output
     • Dyspnea or hemoptysis

2. Consider medication interactions: Methimazole can interact with several medications including:

   • Oral anticoagulants
   • Beta-blockers
   • Digitalis glycosides
   • Theophylline

3. Management options:

   • For mild headaches without other concerning symptoms:
     • NSAIDs may be appropriate for symptomatic relief 3
     • Monitor for resolution
   • For persistent or severe headaches:
     • Consider dose adjustment
     • Evaluate for alternative causes
     • In rare cases, switching to propylthiouracil may be necessary 4
     • In patients with mild reactions (excluding agranulocytosis and hepatotoxicity), desensitization to methimazole under allergist supervision is a potential option 5

Important Monitoring

Patients taking methimazole should be under close surveillance, particularly during the initial weeks of therapy 1. Thyroid function tests should be monitored periodically, typically every 2-4 weeks initially, then every 3-6 months once stable 4.

Differential Diagnosis

When a patient on methimazole presents with headache, consider:

• Direct medication side effect
• Thyroid hormone fluctuations (both hyperthyroidism and rapid correction can cause headaches)
• Unrelated primary headache disorder
• More serious adverse reaction (vasculitis)
• Coincidental headache from another cause

Conclusion

While headache is not among the most common side effects of methimazole, it is a recognized adverse reaction that warrants appropriate evaluation, especially when severe or accompanied by other symptoms that might indicate a more serious reaction.