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There is good evidence that some papillary and follicular thyroid cancers run in families, although such thyroid cancers are still exceedingly rare among the far larger number of non-inherited papillary thyroid cancers. Since there is not yet enough knowledge about the specific genes responsible for the few inherited cases, there is not yet a role for genetic testing. In addition, there are a few unusual genetic syndromes, such as Gardner's syndrome, Carney's complex, and Alagille syndrome, that seem to carry a greater risk of thyroid cancers.

Medullary thyroid cancer is a type of cancer of the thyroid parafollicular cell. Twenty percent of people with this cancer have inherited it from their mother or father. It's been well established that there is a special portion of the tenth chromosome, the RET proto-oncogene, that carries mutations causing this type of thyroid cancer. Both males and females inherit this mutation equally from either parent. Nearly everyone with the mutated gene is virtually guaranteed to get this cancer at some time in their life. In genetic-speak, this is known as an autosomal dominant disorder (meaning it takes only one parent with this mutation to give you the cancer) with 100 percent penetrance (meaning you are 100 percent likely to get the disease if you have this mutated gene).

Families with the inherited form of medullary thyroid cancer have one of three different patterns of inherited disease. The first type, multiple endocrine neoplasia type 2a (MEN2a), is medullary thyroid cancer that is inherited along with a tumor of the adrenal gland that makes adrenaline (pheochromocytoma) and a tumor of the parathyroid gland that results in high calcium levels (hyperparathyroidism). The second type, multiple endocrine neoplasia (MEN2b), is medullary thyroid cancer that is inherited along with pheochromocytoma and unusual body features, such as elongated fingers and toes or bumps on the tongue. The third type is just inherited medullary thyroid cancer without the other associated problems.

If you have a medullary thyroid cancer, it is critical for your physician to send your blood sample to a genetics testing laboratory to see if you have the mutated gene associated with this cancer. If you do (meaning that you test positive for the gene), you need to have your family members tested for the same gene mutation. If other family members have this mutation, they could prevent getting the thyroid cancer by having their thyroid gland completely removed by a surgeon as soon as possible. This procedure is known as a prophylactic (preventive) thyroidectomy. Removing the thyroid gland is necessary because medullary thyroid cancer has no effective treatments once it has spread throughout your body. So a prophylactic thyroidectomy could stop this cancer before it starts, or remove it in an early stage, which could be lifesaving for those affected by this mutation.

For the majority of people with other types of thyroid cancer, there is no evidence of any particular pattern of inheritance, no evidence for previous exposure to cancer-causing radiation, and no known agents that are likely to cause this cancer. Considering the increasing rates of thyroid cancer, however, it seems likely that additional environmental factors, such as some type of pollution or chemical exposure, are responsible for some of these cases.

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