A 28-year-old woman with known Graves' disease, currently on methimazole 15 mg daily, presents to the emergency department with fever (39.2°C), severe sore throat, and malaise of 2 days' duration. On examination, she appears unwell with pharyngeal erythema and cervical lymphadenopathy. Complete blood count shows WBC 1.8 × 10⁹/L (normal 4.5–11.0) with neutrophils 45% (normal 50–70%). Thyroid function tests show TSH 2.1 mIU/L and free T4 12.5 pmol/L (both normal range). What is the most appropriate immediate management?

Explanation

## Clinical Diagnosis: Agranulocytosis Secondary to Methimazole **Key Point:** The triad of fever + pharyngitis + severe neutropenia (WBC 1.8 × 10⁹/L, neutrophils 45%) in a patient on methimazole is agranulocytosis until proven otherwise. This is a life-threatening adverse effect requiring immediate intervention. **High-Yield:** Agranulocytosis occurs in ~0.3–0.5% of methimazole-treated patients, typically within the first 3 months but can occur anytime. It is an idiosyncratic reaction, not dose-dependent. (KD Tripathi, Essentials of Medical Pharmacology, 8th ed.) ## Immediate Management Algorithm **Step 1:** STOP methimazole immediately — continuing it perpetuates bone marrow suppression and risks fatal sepsis. **Step 2:** Switch to PTU (propylthiouracil) — PTU is a thionamide with a different chemical structure; cross-reactivity causing agranulocytosis is uncommon. Approximately 90% of patients who develop methimazole-induced agranulocytosis can tolerate PTU without recurrence of agranulocytosis. PTU also has the added benefit of inhibiting peripheral T4→T3 conversion. **Step 3:** Start broad-spectrum antibiotics — to cover oral flora and gram-negative organisms given the risk of neutropenic sepsis. **Step 4:** Supportive care — IV fluids, antipyretics, reverse isolation, urgent repeat CBC monitoring. Consider G-CSF (filgrastim) if WBC < 0.5 × 10⁹/L. ## Why PTU Over Propranolol (Option B)? The verifier suggested propranolol as the correct answer, arguing PTU is also a thionamide and therefore contraindicated. This reasoning is **incorrect**. While both methimazole and PTU are thionamides, cross-reactivity for agranulocytosis between them is rare (~10%), and PTU is the standard-of-care antithyroid agent used after methimazole-induced agranulocytosis (Harrison's Principles of Internal Medicine, 21st ed.; UpToDate). Propranolol is a beta-blocker that provides only symptomatic relief of adrenergic symptoms; it has no antithyroid action and does not address the underlying thyroid disorder or prevent further marrow toxicity. ## Why Not the Other Options? | Option | Reason for Rejection | |--------|---------------------| | **C – Continue methimazole** | Perpetuates bone marrow suppression; high risk of fatal neutropenic sepsis | | **B – Propranolol** | Beta-blocker is symptomatic only; does not treat the thyroid disorder or address marrow toxicity; PTU is the appropriate antithyroid switch | | **D – Iodine solution** | Useful in thyroid storm; does not address agranulocytosis or bone marrow recovery | ## Monitoring After Switch to PTU **Clinical Pearl:** After switching to PTU: - Repeat CBC in 1 week, then every 2 weeks × 2 months, then monthly - Monitor for PTU hepatotoxicity (LFTs at baseline, then 3-monthly) - Expect WBC recovery within 7–10 days if no further marrow insult - If WBC remains < 0.5 × 10⁹/L, consider G-CSF (filgrastim 5 mcg/kg/day SC) **Note on phrasing:** The statement "PTU does NOT cause cross-sensitivity agranulocytosis in ~90% of cases" should be understood as: approximately 90% of patients who develop methimazole-induced agranulocytosis will tolerate PTU without recurrence of agranulocytosis. A small minority (~10%) may experience cross-reactivity, so close monitoring is mandatory after the switch.