At current rates, one out of 111 Americans will get thyroid cancer at some point during his or her lifetime. And the incidence is increasing, both in the United States and worldwide. Women get thyroid cancer at three times the rate for men. Men and women have the same death rates from this cancer, however. It is highly curable when it is treated early.
One explanation for the rising incidence is improvements in detection and diagnosis. A recent study reports the overall incidence of well-differentiated thyroid cancer in the United States has tripled since 1973. Some of the cancers detected today are small and subclinical, and may have been overlooked in the 1970's. However, the incidence of cancers larger than 4 cm has doubled during this time period. It is not clear what caused this trend.
There are four primary subtypes of thyroid cancer:
1. Papillary carcinoma is the most common type in developed countries, and is the least dangerous.
2. Follicular carcinoma accounts for about 30 percent of cases in the United States, but is more common in developing countries where iodine deficiency is common. It is more aggressive.
3. Anaplastic carcinoma, also called giant or spindle cell cancer, is the most aggressive. It is fortunately rare.
4. Medullary carcinoma is a cancer of non-thyroid cells that are normally present in the thyroid gland.
Papillary and follicular carcinomas together are considered well-differentiated thyroid cancers.
Obesity is one factor associated with the increase in thyroid cancer incidence. A recent study showed that higher body mass index is not associated with more aggressive tumors, just a higher risk of any type thyroid tumors.
Dental X-rays show a small but statistically significant correlation with thyroid cancer. High-dose ionizing radiation is the only established cause, according to Reference 4, so researchers hypothesized that low-dose radiation used for check-ups may also be a risk factor. They recommend further study of diagnostic X-ray exposure.