A 55-year-old woman with anaplastic thyroid carcinoma presented with hyperthyroidism and neck swelling, hoarseness, and cervical lymphadenopathy. On physical examination, she was found to be clinically hyperthyroid with an enlarged, nontender multinodular goitre. Her serum thyroid hormone levels confirmed hyperthyroidism and technetium-99m pertechnetate scan failed to visualize the thyroid gland. Open biopsy showed an invasion of the thyroid gland by anaplastic thyroid carcinoma. The thyrotoxic phase lasted 60 days with predominantly increased thyroxine level and triiodothyronine/thyroxine (T-3/T-4) ratio decreased below 15. The thyrotoxic period was followed by subclinical hyperthyroidism and hypothyroidism which continued until she died of lung metastasis.