About 5% of thyroid nodules that are found during a physical examination in adults are cancerous.¹ Nodules that develop after radiation therapy are more likely to be cancerous. On average, cancer develops 10 to 20 years after radiation therapy. A multinodular goiter is as likely to be cancerous as a single nodule.

Thyroid nodules are less common in children than adults. Although some studies have shown cancerous nodules are more common in children, other studies have found the risk is similar in children and adults.²

There are five types of cancerous nodules.

• Papillary carcinoma. The most common type grows slowly, especially in people who are younger than 45. With treatment, survival rates are very high, even when the cancer has spread to nearby lymph nodes.
• Follicular cancer. This is the second most common type of thyroid cancer. Follicular cancer usually responds well to treatment.
• Medullary carcinoma. This rare type of cancer can be inherited and is harder to treat than papillary carcinoma or follicular cancer. New genetic blood tests can be used to identify people who have a high risk of developing medullary carcinoma. People who test positive, but who do not show any symptoms of thyroid cancer, may choose to have the thyroid gland removed.
• Undifferentiated or anaplastic carcinoma. These rare types of thyroid cancer are typically seen in older people. Usually, by the time a nodule can be felt, the cancer has spread to nearby lymph nodes or other body parts. These tumors are very difficult to treat.
• Thyroid lymphoma. This rare type of lymphoma starts in the thyroid gland. It results in an enlarged thyroid gland rather than in a thyroid nodule. It is more common in people who have Hashimoto's thyroiditis than in people who have thyroid nodules.