The thyroid gland is an endocrine gland located anterior to the trachea and inferior to the thyroid cartilage. It is butterfly shaped with three lobes that make it the largest endocrine organ in the body. Its primary function is to control the body’s metabolism by regulating how quickly the body uses energy, proteins, and hormones.
The two lateral lobes are connected by the middle lobe called the isthmus, which lies directly on top of the trachea. The entire gland is enclosed in a capsule made up of connective tissue called the thyroid cartilage. The thyroid has an irregular surface configuration and is highly vascular with a liberal blood supply. Thyroid tissue is made up of columnar and cuboidal cells that form follicles, which produce thyroid hormones. The follicle cavities are filled with fluid called colloid. The thyroid receives blood from branches of the external carotid artery and the brachiocephalic trunk, and it drains blood via the internal jugular vein and left brachiocephalic vein.
Four parathyroid glands are found near the posterior aspect of the thyroid gland. They are small (20-40 mg) and have a beanlike shape. The superior parathyroid glands are most commonly located in the posterolateral aspect of the superior pole of the thyroid gland at the cricothyroidal cartilage junction. They are most commonly found 1 cm above the intersection of the inferior thyroid artery and the recurrent laryngeal nerve (see the image below). The inferior parathyroid glands are more variable in location and are most commonly found near the lower thyroid pole of the thyroid. The inferior parathyroid gland is supplied by the inferior thyroid artery from the thyrocervical trunk. Inferior parathyroid glands that descend into the anterior mediastinum are usually vascularized by the inferior thyroid artery. The superior parathyroid gland is also usually supplied by the inferior thyroid artery or by an anastomotic branch between the inferior thyroid and the superior thyroid artery.