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Structure

Thyroid gland

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The thyroid is a butterfly-shaped endocrine gland in the anterior neck that produces thyroxine (T4), triiodothyronine (T3) and calcitonin. It is one of the most vascular organs in the body relative to its size.

Terminologia Anatomica (Latin): Glandula thyroidea · Open in the 3D atlas

Location

It lies in front of the trachea at about C5-T1, deep to the sternothyroid and sternohyoid muscles, with two lobes joined by an isthmus overlying the 2nd-4th tracheal rings. A pyramidal lobe, a remnant of the thyroglossal duct, is present in a proportion of people and the gland moves with swallowing.

Structure

The gland comprises follicles lined by follicular cells that store thyroglobulin colloid, with parafollicular C cells producing calcitonin. It is enclosed by a true capsule and the pretracheal fascia. It develops from the foramen caecum at the tongue base, which explains lingual and thyroglossal remnants.

Blood supply

The superior thyroid artery (from the external carotid) and inferior thyroid artery (from the thyrocervical trunk) supply the gland; a thyroid ima artery is present in a minority. Superior and middle thyroid veins drain to the internal jugular vein and the inferior thyroid veins to the brachiocephalic veins. Lymph drains to pretracheal, paratracheal and deep cervical nodes.

Innervation

Sympathetic fibres from the cervical ganglia modulate vascular tone; the gland itself is regulated hormonally by TSH. Important neighbouring nerves are the recurrent laryngeal nerve, which ascends in the tracheo-oesophageal groove close to the inferior thyroid artery, and the external branch of the superior laryngeal nerve, which accompanies the superior thyroid artery.

Function

Thyroid hormones regulate basal metabolic rate, thermogenesis, growth and neurological development, and enhance cardiac responsiveness to catecholamines; secretion is controlled by the hypothalamic-pituitary-thyroid axis (TRH, TSH). Calcitonin has a minor role in calcium homeostasis in humans.

Clinical correlations

Goitre (iodine deficiency is a leading global cause), hyperthyroidism (Graves disease), hypothyroidism (Hashimoto thyroiditis), nodules and carcinoma (papillary most common; medullary arises from C cells) are the main conditions. In thyroidectomy the recurrent laryngeal nerve (hoarseness) and parathyroid glands (hypocalcaemia) are at risk.

Common exam points

  • Superior thyroid artery (external carotid) and inferior thyroid artery (thyrocervical trunk).
  • Recurrent laryngeal nerve lies near the inferior thyroid artery; external laryngeal nerve near the superior thyroid artery.
  • Moves on swallowing because of the pretracheal fascia; thyroglossal cysts also move on tongue protrusion.
  • Isthmus overlies tracheal rings 2-4.
  • Follicular cells: T3/T4; parafollicular C cells: calcitonin.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Williams Textbook of Endocrinology. Elsevier, 14th edition (2019) — Tier 3 (textbook)

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