A 32-year-old woman with Graves' disease is started on antithyroid therapy. Which is the most common adverse effect of propylthiouracil (PTU) that occurs in the first 3 months of treatment?

Explanation

## Most Common Adverse Effect of PTU **Key Point:** Rash and pruritus are the most frequent adverse effects of PTU, occurring in 1–5% of patients, typically within the first 3 months of therapy. ### Frequency and Timing of PTU Adverse Effects | Adverse Effect | Incidence | Timing | Severity | |---|---|---|---| | Rash/pruritus | 1–5% | Early (weeks 1–12) | Mild–moderate; often self-limiting | | Agranulocytosis | 0.1–0.5% | Variable (can be early or late) | Severe; life-threatening | | Hepatotoxicity | 0.1–1% | Variable | Severe; rare but serious | | Hypothyroidism | Common | Late (months 3–12+) | Expected; dose-related | **High-Yield:** While agranulocytosis is the most feared adverse effect, it is NOT the most common. Rash and pruritus are mild, often transient, and do not require drug discontinuation in most cases. **Clinical Pearl:** Patients with rash can often continue PTU with symptomatic management (antihistamines, topical corticosteroids). In contrast, agranulocytosis mandates immediate cessation and hospitalization. ### Why Rash is Most Common - PTU is a thioamide with a thiouracil ring that undergoes hepatic metabolism and can form reactive metabolites. - Hypersensitivity reactions (Type I and IV) are relatively common early manifestations. - Most rashes resolve spontaneously or with symptomatic treatment without stopping the drug. **Warning:** Do NOT confuse "most common" with "most serious." Agranulocytosis is rare but life-threatening; rash is common but benign.