Malignancy of the thyroid, both primary and secondary, is rare, accounting for less than 2% of all cancers, and generally, tumors are not aggressive. Papillary carcinoma is the most common, accounting for approximately 80% of primary thyroid malignancies. Follicular carcinoma, which arises from the same cell type as papillary carcinoma, accounts for about 5% of thyroid neoplasia. Undifferentiated and anaplastic carcinoma makes up less than 10% of all thyroid malignancies and medullary carcinoma of the thyroid about 5% (Baloch and Livolsi, 2005). The most common cancers that metastasize to the thyroid are breast, lung, and kidney. Primary lymphoid cancer occurs in the thyroid, but its incidence is unknown since it cannot be distinguished from lymphoma that originates elsewhere in the body. Treatment of thyroid cancers is generally surgical with 131I therapy after near-total thyroidectomy. As with cancer elsewhere in the body, primary treatment will depend on tissue diagnosis and clinical evaluation.
Two familial thyroid malignancies are multiple endocrine neoplasia type IIA (MEN-IIA) and familial medullary thyroid carcinoma (FMTC). MEN-IIA is an autosomal dominant disorder that can cause "C" cell hyperplasia and hyperparathyroidism. Diagnosis is often serendipitously found during evaluation for hypercalcemia or renal calculi.
Nodules less than 1.0 cm usually do not require FNA, but any solitary nodule occurring before 14 years of age is an exception, with greater than 50% incidence of malignancy.
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