Thyroid gland

At a glance

English

Thyroid gland

Latin (TA2)

Glandula thyreoidea

System

Endocrine gland

Region

Whole body

Side

Single, on the midline

About the thyroid gland

The thyroid gland lies low in the front of the neck, deep to the strap (infrahyoid) muscles and below the thyroid cartilage of the larynx. It has two lateral lobes, one on each side of the larynx and trachea, joined across the front by a narrow isthmus that usually crosses the second to fourth tracheal rings. Each lobe is roughly five centimetres tall, reaching up to the oblique line of the thyroid cartilage and down to about the sixth tracheal ring. A small pyramidal lobe often rises from the isthmus towards the hyoid bone.

A thin fibrous capsule surrounds the gland and sends septa inwards; outside this lies a looser sheath from the pretracheal layer of deep cervical fascia. Thickenings of this sheath, the suspensory ligaments, bind the back of the lobes to the cricoid cartilage and trachea, which is why the thyroid and any swelling of it rise when a person swallows. The carotid sheath lies behind each lobe laterally. The recurrent laryngeal nerve climbs in the groove between trachea and oesophagus close to the inferior thyroid artery, and the parathyroid glands sit on the posterior surface of the lobes.

The gland develops from the floor of the embryonic pharynx at the foramen caecum of the tongue and descends in front of the neck, attached for a time by the thyroglossal duct. Remnants of this duct may form a midline thyroglossal cyst, and thyroid tissue can stop anywhere along its path. Microscopically the gland is made of spherical follicles lined by follicular cells around a store of colloid. These cells take up iodine and make thyroxine (T4) and triiodothyronine (T3), which set the body's metabolic rate and are vital for growth and brain development in children. Scattered parafollicular (C) cells secrete calcitonin, which lowers blood calcium. Output is controlled by thyroid-stimulating hormone from the anterior pituitary, itself driven by the hypothalamus.

Overactivity, usually from Graves disease, causes weight loss, heat intolerance and a rapid pulse; underactivity, from iodine deficiency worldwide or Hashimoto thyroiditis where iodine is plentiful, causes tiredness, weight gain and cold intolerance. Enlargement is goitre. Nodules are common and are checked by ultrasound and needle sampling. During thyroidectomy surgeons protect the recurrent and external laryngeal nerves, whose injury affects the voice, and the parathyroid glands.

Function and supply

Function

Secretes thyroxine and triiodothyronine, which regulate metabolic rate, heat production, growth and development, and calcitonin, which lowers blood calcium.

Nerve supply

Sympathetic fibres from all three cervical ganglia, mainly to the blood vessels, with parasympathetic fibres via the superior and recurrent laryngeal nerves; secretion is controlled chiefly by TSH.

Blood supply

Superior thyroid artery from the external carotid and inferior thyroid artery from the thyrocervical trunk, occasionally a thyroid ima artery; superior and middle thyroid veins empty into the internal jugular, inferior thyroid veins to the brachiocephalic veins.