Certain tumors cells produce hormones and, when the hormone product is functional, may lead to a variety of endocrine disorders. The hormone-producing cells often produce excessive amounts of product, not only due to the rapidly dividing cells, but also due to the lack of negative feedback mechanisms that normally exist to limit hormonal production. The hormone source is considered primary if the endocrine tissue that normally produces the hormone produces hormone in excess. When a hormone is produced at a distant site by non-endocrine tissue, the production is considered ectopic.