Primary thyroid cancer has been divided into well-differentiated thyroid cancer (papillary and follicular thyroid cancer), which comprise approximately 90% of all cases, medullary thyroid cancer, which makes up 5%-9% of cases, and anaplastic or poorly differentiated cancers, which form the remaining 1%-2% of cases. However, various histological subtypes have evolved as we have gained a better understanding of the biology of these cancers. Some subtypes of these tumors have been identified as being more aggressive and thus have been labeled thyroid cancers with intermediate differentiation. These include variants of papillary thyroid cancer, insular carcinoma, and Hürthle cell carcinoma. Insular carcinoma of thyroid is characterized by islands of neoplastic cells that are surrounded by hypocellular fibrous tissue: this cancer has been placed between well-differentiated thyroid carcinoma and anaplastic thyroid carcinoma. A 50-year-old woman suffering from thyroid cancer underwent a transformation from papillary carcinoma to insular carcinoma and poorly-differentiated carcinoma over a 10-year period. A description of the pathology and history of treatment are presented.