The thyroid gland develops embryologically as an epithelial invagination from the base of the foetal tongue. It consists of two large lobes (normally weighing ca. 15-20 g) lying on both sides of the trachea, just beneath the larynx (voice box), connected by a narrow strand of thyroid tissue called the isthmus (Figure 4.1A); a small pyramidal extension of tissue upwards from the isthmus is also sometimes present. The gland receives a profuse blood supply through the thyroid arteries, and is also richly innervated by fibres of the autonomic nervous system, although the role played by these nerves in the control of thyroid blood flow and function seems minimal. The whole gland is enveloped by a capsule of fibrous connective tissue.