ABSTRACT
The thyroid is present in all vertebrates. In humans, it has two lobes, connected by an isthmus, on either side of the trachea. It is enervated by parasympathetic and sympathetic fibers, which control only the diameter of the blood vessels they are in. Marked changes occur in the thyroid with age. Its structure then resembles the gland when it is in a state of hypothyroidism. The ultimobranchial body from endoderm cells of the sixth pharyngeal pouch is incorporated into the thyroid and produces the parafollicular or C cells. Thyroid tissue is present in all vertebrates. The parathyroid glands in the adult human are composed mainly of chief and oxyphil cells. The thymus is a lymphoepithelial endocrine gland located in the mediastinum, and it extends from the thyroid in the neck into the thoracic cavity. Thyroid-stimulating hormone acts by binding to its specific receptor in cell membranes within the thyroid gland.
