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Follicular thyroid cancer is a cancer that forms from the follicular cells of the thyroid and is the second most common type of thyroid cancer. It is more common in females. It usually affects people between the ages of 40 and 60 and can be an aggressive disease, as it characteristically invades blood vessels and spreads (metastasizes) to other parts of the body. A sub-type, Hurthle cell cancer, is a rare, more aggressive type of follicular thyroid cancer. While the exact cause is unclear, thyroid cancer develops when genetic mutations occur in thyroid cells that then reproduce rapidly, form a tumor, and spread to other areas, such as lung, bone, brain, liver, bladder, and skin.