Objective: To describe the clinical presentation, clinical course, and management of a dog with thyroid storm (TS) secondary to a functional thyroid carcinoma. Case summary: A 12-year-old neutered female Golden Retriever was evaluated for severe weight loss, hyperthermia, and tachycardia. The dog had a ventral neck mass and markedly increased thyroxine concentration. Cervical ultrasound showed a suspected left thyroid mass with invasion into the jugular vein. Despite aggressive therapy, the patient progressed to a clinical TS. Postmortem examination was supportive of the clinical diagnosis.