Radiation exposure is a proven risk factor for thyroid cancer.

Sources of such radiation include certain medical treatments and radiation fallout from power plant accidents or nuclear weapons.

The thyroid cancers tend to be either the papillary or follicular type.

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Medullary thyroid cancer: About 2 out of 10 medullary thyroid carcinomas (MTCs) result from inheriting an abnormal gene.

These cases are known as familial medullary thyroid carcinoma (FMTC).

FMTC can occur alone, or it can be seen along with other tumors.

The combination of FMTC and tumors of other endocrine glands is called multiple endocrine neoplasia type 2 (MEN 2).

Cowden disease: People with this syndrome have an increased risk of thyroid problems and certain benign growths (including some called hamartomas).

They also have an increased risk of cancers of the thyroid, uterus, breast, as well as some others.

The papillary type of thyroid cancer most often runs in families.

Genes on chromosome 19 and chromosome 1 are suspected of causing these familial cancers.

This syndrome is caused by defects in the gene PRKAR1A.

Familial nonmedullary thyroid carcinoma: Thyroid cancer occurs more often in some families, and is often seen at an earlier age.

For unclear reasons thyroid cancers (like almost all diseases of the thyroid) occur about 3 times more often in women than in men.