BACKGROUND Papillary thyroid carcinoma is usually an indolent disease, with an almost 80% 5-year survival rate for metastatic disease. Conversely, anaplastic thyroid cancer is much more aggressive, with median overall survival rates of 4 months. CASE REPORT A 67-year-old woman presented with metastatic papillary thyroid cancer with bone metastasis, including an unstable L4 pathological fracture. Initially, she underwent lumbar stabilization surgery, followed by high-dose palliative radiotherapy to the lumbar spine. Subsequently, a total thyroidectomy was performed, followed by an ablative dose of radioiodine and supraphysiological doses of levothyroxine to achieve TSH suppression to less than 0.1 mU/L. The treatment dose of radioiodine was administered 4 times at 6-month intervals. The treatment was well tolerated, with a dramatic thyroglobulin response, and the disease remained radioiodine-sensitive. Prior to a fifth planned dose of radioiodine, our patient presented with cauda equina syndrome and underwent urgent decompressive surgery. Further oncological treatment was planned; however, she deteriorated rapidly following surgery, and repeat imaging showed progressive disease at the surgical site. Histopathology from the lumbar decompression revealed anaplastic thyroid cancer. Our patient died 5 weeks after surgery. CONCLUSIONS This is the first published case of transformation from papillary to anaplastic thyroid cancer presenting as cauda equina compression. Transformation from papillary to anaplastic thyroid cancer has been previously described in the literature; however, it is rarely present distant from the neck, and has an aggressive course. Malignant transformation should be considered in cases of differentiated thyroid cancer that do not fit the previous disease trajectory.