The thyroid gland is the body's largest specialised endocrine gland of the human body. By producing several thyroid hormones (e.g. thyroxine (T4), triiodothyronine (T3) and calcitonin) it plays a fundamental role for development and metabolism.
From evolutionary perspective, the thyroid gland derives from the epithelium of the hyoid arch. At the beginning of embryonic development, it still is connected to the base of the mouth via the thyroglossal duct. It assumes its final position after its descensus caudal and ventral to the thyroid cartilage in front of the trachea.
The oral region, from where the thyroid gland has descended, is recognizable on the surface of the tongue as foramen caecum. In some cases, a connection of the thyroid gland and the oral region might be remaining. This so-called pyramidal lobe is located ventral to the trachea, and can present a risk factor in trachectomy for unexpectedly strong hemorrhage.
They both are directly under the trachea on its ventral and lateral side.
Each thyroid lobe has about the dimensions of a bigger olive. The average total volume amounts to around 18 mL in women, and 25 mL in men. The lobes are surrounded by two capsules of connective tissue. The exterior fibrous capsule connects the thyroid gland with the trachea, the interior serous capsule is directly connected to the thyroid tissue. Between the two capsules, there are the parthyroid glands on the dorsal side.
The thyroid gland is located below the pretracheal fascia, approximately on the height of C6 to C7. It is ventral to the trachea on the height of the 2nd to 4th cricoid cartilage.
Dorsomedial to the thyroid gland, there is the recurrent laryngeal nerve, lateral to it you can find the carotid sheath (risk of injury during goiter surgery). On its ventrolateral side, the thyroid gland is covered by the sternohyoid muscle and the sternothyroid muscle, on the lateral edge, there is the sternocleidomastoideus muscle.
The upper part of the thyroid gland is supplied with blood via the superior thyroid artery, a branch of the external carotid artery. The lower part receives blood from the inferior thyroid artery, which originates in the thyreocervical trunk. Both vessels form numerous anastomoses among each other. Around 5% of all people have an additional source of supply through the thyroid ima artery, a direct branch of the brachiocephalic trunk.
The venous blood of the thyroid gland flows back into the internal jugular vein via the superior thyroid vein and the middle thyroid vein, and into the brachiocephalic vein via the inferior thyroid vein.
The innervation of the thyroid gland is provided by the nerve fibers of the vegetative nervous system. The sympathetic fibers stem from the superior cervical ganglion, or, according to some other books, from the middle cervical ganglion. The parasympathetic supply is provided by the laryngeal branches of the vagus nerve (Xth cranial nerve).
The thyroid produces different classes of thyroid hormones, which include iodine-containing iodothyronines (e.g. T3, T4 and 3,5-T2), iodothyroacetates and thyronamines (all of them significantly impacting the metabolism) as well as the peptide hormone calcitonin (a minor regulator of calcium homeostasis).
Production and release of thyroid hormones is controlled by multiple feedback control systems, where thyrotropin (TSH), a glycoprotein hormone produced by the pituitary gland, is the main regulator of thyroid function.
The enlargement of the thyroid gland, medically called goiter ("struma") can be prevented by an adequate iodine supply of the population. In countries with drinking water iodination, such as Switzerland and Sweden, the incidence of thyroid diseases is lower. Especially women are affected by iodine deficiency.
The named diseases can concur with different functional states of the thyrotropic feedback control.