anteriorly in the lower neck, level with the fifth cervical to the first thoracic vertebrae (see Fig. 29.17). It is ensheathed by the pretracheal layer of deep cervical fascia and consists of right and left lobes connected by a narrow, median isthmus. It usually weighs 25 g but this varies. The gland is slightly heavier in females and enlarges during menstruation and pregnancy. Estimation of the size of the thyroid gland is clinically important in the evaluation and management of thyroid disorders and can be achieved non-invasively by means of diagnostic ultrasound. Mean thyroid volume increases with age (Chanoine et al 1991). No significant difference in thyroid gland volume has been observed between males and females from 8 months to 15 years. The lobes of the thyroid gland are approximately conical. Their ascending apices diverge laterally to the level of the oblique lines on the laminae of the thyroid cartilage, and their bases are level with the fourth or fifth tracheal cartilages. Each lobe is usually 5 cm long, its greatest transverse and anteroposterior extents being 3 cm and 2 cm, respectively. The posteromedial aspects of the lobes are attached to the side of the cricoid cartilage by a lateral thyroid ligament (Berry’s ligament). The isthmus connects the lower parts of the two lobes, although occasionally it may be absent. It measures 1.25 cm transversely and vertically, and is usually anterior to the second and third tracheal cartilages, although it can be higher or even sometimes lower because its site and size vary greatly. A conical pyramidal lobe often ascends towards the hyoid bone from the isthmus or the adjacent part of either lobe (more often the left). It is occasionally detached or in two or more parts. A fibrous or fibromuscular band, the levator of the thyroid gland, musculus levator glandulae thyroideae, sometimes descends from the body of the hyoid to the isthmus or pyramidal lobe. For further reading, see Mohebati and Shaha (2012). Ectopic thyroid tissue is rare but may be found around the course of the thyroglossal duct or laterally in the neck, as well as in distant places such as the tongue (lingual thyroid), mediastinum and the subdiaphragmatic organs (Noussios et al 2011). The most frequent location of ectopic thyroid tissue is at the base of the tongue, in particular at the region of the foramen caecum; often it is the only thyroid tissue present. Small, detached masses of thyroid tissue may occur above the lobes or isthmus as accessory thyroid glands. Vestiges of the thyroglossal duct may persist between the isthmus and the foramen caecum of the tongue, sometimes as accessory nodules or cysts of thyroid tissue near the midline or even in the tongue, where they are called thyroglossal duct cyst.PARATHYROID GLANDS The parathyroid glands are small, yellowish-brown, ovoid or lentiform structures, usually lying between the posterior lobar borders of the thyroid gland and its capsule. They are commonly 6 mm long, 3–4 mm Neck 472SECTION 4 across and 1–2 mm from back to front, each weighing about 50 mg. Typically, there are two on each side, superior and inferior, but there may be more or there may be only three or many minute parathyroid islands scattered in connective tissue near the usual sites. Very occasionally, an occult gland may follow a blood vessel into a groove on the surface of the thyroid. Normally, the inferior parathyroids migrate only to the inferior thyroid poles, but they may descend with the thymus into the thorax or they may be sessile and remain above their normal level near the carotid bifurcation. The anastomotic connection between the superior and inferior thyroid arteries that occurs along the posterior border of the thyroid gland usually passes very close to the parathyroids, and is a useful aid to their identification. The superior parathyroid glands are more constant.