Well differentiated thyroid cancer is unique in that age is a key factor in the expected course of the disease, independent of other characteristics. There are eight staging systems reported in Reference 1, of which seven use age as a prognostic factor. A report from the University of Wisconsin suggests that the disease mechanism may vary with age, and treatment program should take this difference into account.
There are six theories proposed for the association between age and thyroid cancer prognosis:
1. Iodine uptake. Most of the iodine consumed in the diet is absorbed by normal thyroid tissue, and used to produce thyroid hormones. Thus, radioactive iodine can be used to selectively irradiate thyroid tissue. Standard therapy for thyroid cancer includes the use of radioiodine in high enough doses to kill all the cells that absorb it. For patients younger than 20 years, thyroid tumors and metastases take up iodine at approximately the same rate as normal thyroid tissue. Thus, treatment destroys the cancer effectively. With age, the iodine uptake of thyroid tumors decreases. The reasons for this are not entirely clear.
2. Genetic variation. There are multiple genes that may be involved in the development of cancer. Age may be associated with mutations in different genes. This may be the cause of different radioiodine sensitivities and differences in tumor aggressiveness.
3. Thyroid stimulating hormone (TSH) levels increase with age, and higher TSH levels are associated with higher risk for thyroid cancer.
4. For women, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) increase around the time of menopause. These hormones have part of their chemical structure in common with TSH, so they may have similar effects on the thyroid.
5. Immune system decline. Current research suggests an important role for the immune system in fighting cancer in general, and immune function typically declines with age. Thyroid cancer may be one of the most sensitive to immune control.
6. Overall mortality. This is a factor for all cancers. Older people have a higher mortality in general, so it is important to consider age-adjusted statistics.