Methimazole and Dull Headache
Yes, methimazole can cause headache as a recognized adverse effect, and patients should be instructed to report headaches promptly as part of routine monitoring for potential complications. 1
FDA-Labeled Adverse Effect
The FDA prescribing information for methimazole explicitly lists headache among the symptoms that warrant immediate reporting and clinical evaluation. 1 Patients receiving methimazole should be under close surveillance and cautioned to report immediately any evidence of illness, particularly:
- Sore throat
- Skin eruptions
- Fever
- Headache
- General malaise
Clinical Significance and Monitoring
When headache occurs in a patient taking methimazole, it requires prompt evaluation to distinguish between a benign side effect and a warning sign of serious complications. 1 The concern is that headache may be an early manifestation of more serious adverse reactions, including:
- Agranulocytosis (affects 3 per 10,000 patients) - headache may accompany fever and sore throat 2
- Vasculitis - occurs in less than 5% of cases and can present with various systemic symptoms 2, 1
- Hepatotoxicity - headache may be part of a constellation of symptoms including general malaise 2
Recommended Clinical Response
When a patient on methimazole reports headache, obtain a complete blood count with differential to rule out agranulocytosis, even if the headache seems benign. 1 The FDA label specifically states that white blood cell and differential counts should be obtained when patients report symptoms of illness to determine whether agranulocytosis has developed. 1
Additional evaluation should include:
- Assessment for fever, sore throat, or other signs of infection 1
- Evaluation for signs of vasculitis (new rash, hematuria, decreased urine output, dyspnea) 1
- Liver function tests if hepatotoxicity is suspected 2
- Thyroid function tests, as symptoms may relate to inadequate thyroid control 1
Important Caveat
The majority of methimazole adverse effects, including headache, typically develop within the first 3 months of therapy. 3 However, vigilance should be maintained throughout the treatment course, as serious complications can occur at any time during therapy. 1