Thyroid

The thyroid is an endocrine gland found in vertebrates. In humans, it sits in the neck, just below the Adam’s apple, and is shaped like a butterfly or the letter H. It has two connected lobes, and the lower two-thirds of these lobes are linked by a thin strip of tissue called the isthmus. Under a microscope, the gland’s basic working unit is the spherical thyroid follicle. This follicle is lined with follicular cells (also called thyrocytes) and occasional parafollicular cells, which surround a central space filled with colloid.

The gland releases three hormones: two thyroid hormones—triiodothyronine (T3) and thyroxine (T4)—plus a peptide hormone called calcitonin. T3 and T4 control the body’s metabolic rate, help regulate protein synthesis, and are essential for growth and development in children. Calcitonin helps manage calcium levels in the body.

The release of T3 and T4 is controlled by thyroid-stimulating hormone (TSH), which comes from the anterior pituitary gland. TSH itself is regulated by thyrotropin-releasing hormone (TRH), made by the hypothalamus.

Various disorders can affect the thyroid. Hyperthyroidism means too much hormone is produced; the most common cause is Graves’ disease, an autoimmune condition. Hypothyroidism means too little hormone is produced; the most common cause globally is iodine deficiency. In areas where iodine is scarce, this deficiency is the leading preventable cause of intellectual disability in children. Where iodine is plentiful, the most common cause of hypothyroidism is Hashimoto’s thyroiditis, another autoimmune disorder. Other thyroid problems include inflammation (thyroiditis), enlargement (goitre), nodules, and cancer.

**Structure**

The gland consists of a left and a right lobe, connected by a narrow band called the isthmus. In adults, it weighs about 25 grams; each lobe is roughly 5 cm long, 3 cm wide, and 2 cm thick, while the isthmus is about 1.25 cm in both height and width. The gland is typically larger in women than in men and grows during pregnancy. It sits near the front of the neck, against and around the front of the larynx and trachea. The thyroid cartilage and cricoid cartilage lie just above it, below the Adam’s apple. The isthmus spans from the second to the third rings of the trachea, while the top of the lobes reaches the thyroid cartilage and the bottom extends to around the fourth to sixth tracheal rings. The infrahyoid muscles are in front of the gland, and the sternocleidomastoid muscle lies to the side. Behind the outer wings of the thyroid are the two carotid arteries. The trachea, larynx, lower pharynx, and esophagus all sit behind the gland. In this area, the recurrent laryngeal nerve and the inferior thyroid artery run next to or through the ligament. Typically, four parathyroid glands (two on each side) lie between the two layers of the thyroid capsule, at the back of the thyroid lobes.

A thin fibrous capsule covers the thyroid. This capsule has an inner and an outer layer. The inner layer pushes into the gland, forming septa that divide the tissue into microscopic lobules. The outer layer connects to the pretracheal fascia, and a thickening of this fascia—called the posterior suspensory ligament of the thyroid, or Berry’s ligament—attaches the gland to the cricoid and thyroid cartilages. This attachment makes the thyroid move up and down when a person swallows.

**Blood, lymph, and nerve supply**

Arterial blood reaches the thyroid through the superior thyroid artery (a branch of the external carotid artery) and the inferior thyroid artery (a branch of the thyrocervical trunk). Sometimes an anatomical variant, the thyroid ima artery, also supplies it. The superior thyroid artery splits into anterior and posterior branches, and the inferior thyroid artery splits into superior and inferior branches; these arteries join behind the outer part of the lobes. Venous blood drains through the superior and middle thyroid veins into the internal jugular vein, and through the inferior thyroid veins, which start in a network of veins and empty into the left and right brachiocephalic veins. Both arteries and veins form a plexus between the two layers of the thyroid capsule.

Lymphatic drainage often passes through the prelaryngeal lymph nodes (just above the isthmus) and the pretracheal and paratracheal lymph nodes. The gland receives sympathetic nerve supply from the superior, middle, and inferior cervical ganglia of the sympathetic trunk, and parasympathetic nerve supply from the superior laryngeal nerve and the recurrent laryngeal nerve.

**Variation**

The size and shape of the thyroid, as well as the position of the embedded parathyroid glands, can vary. Sometimes a third lobe, called the pyramidal lobe (or Lalouette’s pyramid), is present. When it exists, it often stretches up to the hyoid bone from the isthmus and may appear as one or several divided lobes. Studies report its presence in 18.3% to 44.6% of people; it more often arises from the left side and can be separate. The pyramidal lobe is a remnant of the thyroglossal duct, which normally disappears during the gland’s descent. Small accessory thyroid glands can occur anywhere along that duct, from the foramen cecum of the tongue to the adult position of the thyroid. A small horn at the back of the thyroid lobes, usually near the recurrent laryngeal nerve and the inferior thyroid artery, is called Zuckerkandl’s tubercle. Other variants include a levator muscle of the thyroid gland.