Antithyroid drugs (ATDs) such as methimazole and propylthiouracil are mainstays in the management of hyperthyroidism but may rarely cause agranulocytosis, a potentially life-threatening hematologic complication. Although international studies have characterized this condition, local data on its presentation, risk factors, and outcomes among Filipino patients are limited. This study aimed to describe the clinicodemographic profile, outcomes, and risk factors associated with ATD-induced agranulocytosis among adult hyperthyroid patients admitted at Baguio General Hospital and Medical Center (BGHMC) from January 2014 to December 2024.

A cross-sectional review of medical records was conducted among adult patients (>18 years old) diagnosed with hyperthyroidism who developed agranulocytosis secondary to ATD use. Patients with malignancy on active chemotherapy or neutropenia unrelated to ATDs were excluded. Data on demographics, comorbidities, drug type, dosage, treatment duration, and outcomes were analyzed using descriptive statistics and logistic regression.

Of 479 hyperthyroid admissions, 21 (4.4%) developed ATD-induced agranulocytosis. The mean age was 41 years, and 81% were female. Fever and sore throat were the most frequent presenting symptoms, while 85.7% had an absolute neutrophil count below 200/μL. Methimazole accounted for 95% of cases, with 80.9% receiving ≥40 mg/day. Agranulocytosis developed within three months of therapy in 81% of patients. Fifteen (71%) survived and were discharged improved, while six (29%) died—mostly from infectious complications. No significant associations were found between mortality and demographic or treatment variables.

ATD-induced agranulocytosis, though uncommon, remains a serious adverse effect, particularly during early, high-dose methimazole therapy. Infection remains the leading cause of death. Vigilant monitoring during the initial treatment months, patient education on early warning signs, and prompt discontinuation of ATDs at symptom onset are vital to improving survival.