any of various benign tumours (adenomas) or malignant tumours (cancers) of the thyroid gland. Most thyroid tumours are adenomas; these have a wide variation of cellular patterns. Most of the tumours have well-developed follicles; thus, they are collectively known as follicular adenomas. Most thyroid adenomas do not take up iodine or produce thyroid hormone. Thyroid cancer is relatively rare—only about 5 percent of all thyroid nodules are cancers, and only about 0.5 percent of all cancer deaths are from thyroid cancer. The peak incidence of thyroid cancer occurs at about 50 years of age, and women are affected about three times as often as men. Most thyroid cancers are composed of mature-looking thyroid cells, grow very slowly, and are rarely fatal.

The main manifestation of a benign or malignant thyroid tumour is a painless mass in the neck. The diagnosis of a nodule is confirmed by ultrasonography (ultrasound). The only reliable way to distinguish between benign and malignant nodules is by fine-needle aspiration of cells (cell), the characteristics of which are examined by a pathologist; however, sometimes this distinction can be made only by study of the entire nodule. Patients with nodules that are suspected to be malignant are treated by surgery and often with radioactive iodine, followed by treatment with thyroxine.