This report concerns a 71-year-old female with long standing untreated toxic multinodular goiter (TMG) coexisting with anaplastic thyroid carcinoma who presented with progressive shortness of breath and easy fatigability. Thyroid function tests showed suppressed TSH and elevated FT4 and FT3. Anti-thyroid hormone drugs were started. Thyrotropin receptor antibody was negative. A thyroid scan revealed both hot and cold nodules and fine needle aspiration biopsy (FNAB) of the thyroid gland showed colloid nodule. Chest CT scan imaging revealed an enlarged thyroid gland with the right lobe compressing the trachea. Patient underwent total thyroidectomy, final histopathology showed Anaplastic thyroid carcinoma. Our case is interesting from a pathophysiologic perspective,since it suggests that TMG can potentially transform into an aggressive form of thyroid carcinoma. High index of suspicion in patients with TMG with other risk factors for malignancy requires careful evaluation to detect cancer.