There are four types of thyroid cancer according to their cytology: 1) papillary thyroid cancer, which accounts for about 75%; 2) follicular thyroid cancer, which accounts for about 16%; 3) medullary thyroid cancer, which accounts for about 5%; and 4) undifferentiated thyroid cancer, which accounts for about 3%. Papillary thyroid cancer and follicular thyroid cancer, also known as differentiated thyroid cancer, are less malignant, while medullary thyroid cancer and undifferentiated thyroid cancer, known as poorly differentiated thyroid cancer, are more malignant. The remaining 1% are other malignant thyroid tumors, such as thyroid lymphoma, metastatic thyroid cancer and squamous thyroid cancer, which are very rare clinically and have a higher malignancy.