Anaplastic thyroid carcinoma (ATC) is one of the most aggressive solid tumours. It generally presents as a rapidly enlarging thyroid mass and produces local symptoms associated with mass effect. One of the very rare presentations of ATC is thyrotoxicosis. We report a patient with ATC whose course was complicated by severe thyrotoxicosis. His symptoms were controlled with beta-blockers. Two weeks into hospitalization, the patient became hypothyroid. Histopathology showed destruction of the normal thyroid follicles by the invasion of the tumour. As this form of thyrotoxicosis resembles various thyroiditides, we hereby refer this condition as "anaplastic pseudothyroiditis".