About Hyperthyroidism. The thyroid gland is a part of the body's endocrine (hormone) system â which is made up of the glands that help regulate many of the body's different functions. An overactive thyroid produces too much of the thyroid hormone thyroxine, which causes some of these functions to speed up.Hyperthyroidism is the medical term for an overactive thyroid gland. Too little thyroxine reverses the process and slows down the body's functions.hyroid hormone is made by the thyroid gland and can also be made in the laboratory. Thyroid-Stimulating Hormone (TSH). A hormone produced by the pituitary gland. TSH stimulates the release of thyroid hormone.