Thyroi cancer refers to any of four kinds of malignant tumors of the thyroid gland: papillary, follicular, medullary or anaplastic. Papillary and follicular tumors are the most common. They grow slowly and may recur, but are generally not fatal in patients under 45 years of age. Medullary tumors have a good prognosis if restricted to the thyroid gland and a poorer prognosis if metastasis occurs. Anaplastic tumors are fast-growing and respond poorly to therapy.