Severe Neutropenia Due to Carbimazole and the Importance of Early Recognition and Management

Agranulocytosis is a rare but potentially life-threatening adverse effect of antithyroid medications, particularly carbimazole. This case report presents a well-structured and clinically relevant discussion of a rare but serious adverse effect of carbimazole therapy in a patient with Graves' disease. The case highlights the importance of early recognition and prompt discontinuation of the drug, which are crucial to preventing severe complications. We present the case of a 51-year-old female patient who was recently diagnosed with Graves' disease and developed agranulocytosis, leading to neutropenic sepsis after initiating carbimazole therapy. She initially presented to the medical same-day emergency care (SDEC) unit five weeks earlier with complaints of palpitations for two months, accompanied by weight loss, heat intolerance, irritability, and excessive sweating. Her thyroid function tests revealed a thyroxine (T4) level of 61.1 pmol/L and a thyroid-stimulating hormone (TSH) level of <0.01 mIU/L. An electrocardiogram (ECG) showed sinus tachycardia, while a thyroid ultrasound indicated an enlarged thyroid gland consistent with thyroiditis, along with a single U2 nodule in the right lobe. She was prescribed carbimazole and propranolol and was referred for outpatient endocrinology follow-up. After 4-5 weeks of treatment, she developed a fever, sore throat, and fatigue. A routine blood test ordered by her general practitioner (GP) revealed severe neutropenia, consistent with agranulocytosis. Carbimazole was immediately discontinued, and she was treated with broad-spectrum antibiotics and granulocyte colony-stimulating factor (G-CSF), leading to a gradual recovery. Carbimazole-induced agranulocytosis typically occurs within the first few months of therapy. Early discontinuation of the drug is associated with a favorable prognosis. Alternative treatment options, including radioactive iodine therapy and thyroidectomy, should be considered for long-term disease management.