A 38-year-old woman with Graves' disease is treated with antithyroid drugs. She develops a rash and mild fever on day 5 of therapy. Which feature best distinguishes an allergic reaction to methimazole from one to PTU?

Explanation

## Allergic Reactions: PTU vs Methimazole **Key Point:** The feature that **best distinguishes** an allergic/adverse reaction to PTU from one to methimazole is that **PTU** is classically associated with **ANCA-positive vasculitis and glomerulonephritis**, a serious systemic complication that is not a recognized distinguishing feature of methimazole. Methimazole, by contrast, is associated with simple cutaneous reactions (urticaria, maculopapular rash) as its predominant allergic manifestation. ### Adverse Reaction Profile Comparison | Adverse Effect | PTU | Methimazole | |---|---|---| | **Cutaneous reactions** | Rash, urticaria | Rash, urticaria (common) | | **ANCA+ vasculitis / GN** | **Yes — well-documented** | Rare / not a distinguishing feature | | **Hepatotoxicity** | Yes (fulminant hepatic failure reported) | Rare (cholestatic pattern, milder) | | **Agranulocytosis** | 0.1–0.5% | 0.01–0.1% | | **Timing of onset** | Variable | Variable | **Why Option B is correct:** PTU-induced ANCA-positive vasculitis (MPO-ANCA) and glomerulonephritis is a well-established, textbook-level distinguishing adverse effect of PTU that is **not** a characteristic feature of methimazole. This is the single best differentiating feature between the two drugs' allergic/immune-mediated reactions. (KD Tripathi, *Essentials of Medical Pharmacology*, 8th ed.; Harrison's *Principles of Internal Medicine*, 21st ed.) **Why Option C is less accurate as the "best distinguishing" feature:** While PTU can cause hepatotoxicity and methimazole's reactions are more cutaneous, **both** drugs can cause hepatic injury (methimazole causes cholestatic hepatitis, albeit less commonly). Therefore, hepatotoxicity does not *exclusively* distinguish PTU from methimazole. ANCA-positive vasculitis/GN is a more specific and reliable differentiator. **Why Options A and D are incorrect:** - Option A is factually wrong — PTU reactions do not resolve spontaneously without drug discontinuation. - Option D's timing claims (48 hours vs 1 week) lack strong pharmacological evidence and are not standard teaching. **High-Yield:** PTU → ANCA-positive vasculitis/glomerulonephritis (MPO-ANCA). This is a classic exam distinguisher. Always check ANCA and urinalysis in a patient on PTU who develops systemic symptoms. **Clinical Pearl:** PTU is reserved for the first trimester of pregnancy and thyroid storm. Its immune-mediated adverse effects (ANCA vasculitis, hepatotoxicity) make it a second-line agent in most other settings. Methimazole's adverse reactions are predominantly cutaneous and generally milder. **Mnemonic:** **PTU = ANCA Vasculitis** (systemic); **Methimazole = Mild skin** (cutaneous).