Origins, differentiation and spread of thyroid malignancies
THE CONTEXT
Primary thyroid malignancies predominantly arise from follicular cells, but other cellular origins and secondary involvement of the thyroid also occur.
Primary malignancies
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The vast majority of primary thyroid malignancies are carcinomas derived from follicular cells.
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The traditional differentiated group includes papillary, follicular and Hürthle cell carcinomas; anaplastic carcinoma belongs to the undifferentiated group.
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Poorly differentiated carcinoma is an intermediate category, likely representing a state of dedifferentiation between classic differentiated and undifferentiated disease.
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Malignant transformation of parafollicular C cells produces medullary carcinoma. Thyroid lymphoma is another primary thyroid malignancy.
Secondary involvement and metastatic presentation
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The thyroid may be involved by direct spread from the larynx or oesophagus.
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Metastatic involvement of the thyroid most commonly originates from renal cell carcinoma.
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Thyroid cancer can spread through lymphatic and blood-borne routes. Bone and lung are principal metastatic sites, and metastases may be the presenting feature.