Thyroid gland

Glandula thyroidea

Also known as: thyroid

A butterfly-shaped gland (≈15–25 g) in the anterior neck that stores iodinated thyroglobulin in follicles and secretes thyroxine (T4) and triiodothyronine (T3), which set metabolic rate, plus calcitonin from its C cells.

Show on body map Hypothalamic–Pituitary Axis

Overview

Mass≈15–25 g

Lobe height≈5 cm

Hormone secreted≈90 % T4, ≈10 % T3

Daily iodine requirement≈150 µg (250 µg in pregnancy/lactation)

The thyroid has two lateral lobes (≈5 cm tall) joined by an isthmus that lies over the 2nd–4th tracheal rings; in about half of people a pyramidal lobe projects upward from the isthmus towards the hyoid, a remnant of the thyroglossal duct. It is enclosed in a thin fibrous true capsule and, outside that, in the pretracheal layer of deep cervical fascia, which attaches it to the larynx and trachea (the posterior suspensory ligament of Berry binds it to the cricoid). This is why the thyroid and any goitre or nodule moves upward on swallowing.

It is the largest purely endocrine gland and the only one that stores large amounts of hormone extracellularly: about 2–3 months' supply of hormone sits in follicular colloid as thyroglobulin. Follicular cells take up iodide, iodinate tyrosyl residues of thyroglobulin and release T4 and T3 under the control of TSH. Scattered parafollicular (C) cells secrete calcitonin. The gland has a very rich blood supply (≈4–6 mL/min/g), which is why a hyperactive goitre may have an audible bruit.

Location: Anterior neck, deep to the infrahyoid (strap) muscles, at vertebral levels ≈C5–T1; lobes extend from the oblique line of the thyroid cartilage to about the 6th tracheal ring.

Parts & subdivisions

Right and left lobes
Pyramidal lobes with upper and lower poles, wrapping the larynx and trachea.
Isthmus
Crosses the 2nd–4th tracheal rings; divided in tracheostomy.
Pyramidal lobe
Present in ≈30–50 %; may be anchored to the hyoid by the levator glandulae thyroideae.
Follicles
Spherical epithelial sacs filled with colloid — the functional units.
Parafollicular (C) cells
Neural-crest-derived calcitonin-secreting cells between follicles.

Anatomy

Innervation: Sympathetic vasomotor fibres from the superior, middle and inferior cervical ganglia; secretion is hormonally (TSH), not neurally, controlled.
Arterial supply: Superior thyroid artery (first branch of the external carotid) to the upper pole, accompanied by the external branch of the superior laryngeal nerve; inferior thyroid artery (thyrocervical trunk of the subclavian) to the lower pole and posterior surface, closely related to the recurrent laryngeal nerve; inconstant thyroid ima artery (≈10 %, from the brachiocephalic trunk or aortic arch) ascending on the trachea.
Venous drainage: Superior and middle thyroid veins → internal jugular vein; inferior thyroid veins → brachiocephalic veins (mainly left), forming a plexus anterior to the trachea (bleeding risk in tracheostomy).
Lymphatic drainage: Prelaryngeal (Delphian), pretracheal and paratracheal nodes → deep cervical nodes.
Relations: Anterior: sternothyroid, sternohyoid and omohyoid (strap) muscles, sternocleidomastoid. Medial: larynx, trachea, pharynx and oesophagus, with the recurrent laryngeal nerves in the tracheo-oesophageal grooves. Posterolateral: carotid sheath (common carotid artery, internal jugular vein, vagus nerve). Posterior: parathyroid glands.
Histology: Follicles of simple epithelium (low cuboidal when inactive, tall columnar when TSH-stimulated) surrounding eosinophilic colloid; C cells lie singly or in clusters within the follicular basal lamina; dense fenestrated capillary network between follicles.
Development: Endodermal thyroid diverticulum at the foramen caecum of the tongue (week 3–4), descending via the thyroglossal duct to the front of the trachea by week 7; C cells from neural crest via the ultimobranchial bodies. Begins to concentrate iodine and make hormone at ≈10–12 weeks.

Function

Synthesises, stores and secretes thyroxine (T4) and triiodothyronine (T3).
Thereby regulates basal metabolic rate, thermogenesis, growth and nervous-system development.
Secretes calcitonin from C cells.

Regulation: TSH from the anterior pituitary (stimulates every step from iodide uptake to hormone release and causes hypertrophy/hyperplasia); iodide availability (excess acutely inhibits organification — Wolff–Chaikoff effect).

Clinical significance

Goitre

Any enlargement of the thyroid. Diffuse goitre: iodine deficiency (TSH-driven hyperplasia), Graves disease, Hashimoto thyroiditis, puberty/pregnancy. Multinodular goitre may become toxic. Large retrosternal goitres compress the trachea (stridor), oesophagus or great veins (Pemberton sign).

Thyroid nodules and cancer

Nodules are common; most are benign. Papillary carcinoma (≈80 %; "Orphan Annie" nuclei, psammoma bodies, lymphatic spread, excellent prognosis; linked to radiation), follicular (haematogenous spread to bone/lung), medullary (C cells, calcitonin, amyloid stroma, MEN2/RET), anaplastic (elderly, very aggressive). Assess with TSH, ultrasound and fine-needle aspiration.

Thyroidectomy complications

Haemorrhage compressing the airway; recurrent laryngeal nerve injury (unilateral → hoarseness; bilateral → stridor and airway obstruction); external branch of the superior laryngeal nerve injury (loss of voice pitch, cricothyroid paralysis); hypocalcaemia from parathyroid removal or devascularisation; hypothyroidism.

Thyroglossal duct cyst

The commonest congenital midline neck mass, usually near the hyoid; it moves up with tongue protrusion. Excision includes the middle of the hyoid body (Sistrunk procedure) to prevent recurrence.

Test yourself

During thyroidectomy, ligation of which artery places the recurrent laryngeal nerve at greatest risk?

Superior thyroid artery
Inferior thyroid artery
Thyroid ima artery
Lingual artery

Answer: Inferior thyroid artery

The recurrent laryngeal nerve ascends in the tracheo-oesophageal groove and is intimately related to the branches of the inferior thyroid artery near the lower pole. The external branch of the superior laryngeal nerve accompanies the superior thyroid artery instead.

A patient has TSH 18 mU/L (high) and a low free T4. Where is the lesion?

Hypothalamus (tertiary hypothyroidism)
Pituitary (secondary hypothyroidism)
Thyroid gland (primary hypothyroidism)
Peripheral thyroid hormone resistance only

Answer: Thyroid gland (primary hypothyroidism)

With an intact pituitary, falling T4 removes negative feedback and TSH rises. A low T4 with low or inappropriately normal TSH would point to pituitary or hypothalamic disease.

What is the mechanism of hyperthyroidism in Graves disease?

Anti-TPO antibodies destroying follicles
IgG antibodies that stimulate the TSH receptor
A TSH-secreting pituitary adenoma
Release of preformed hormone from inflamed follicles

Answer: IgG antibodies that stimulate the TSH receptor

TSH-receptor-stimulating antibodies (TSI/TRAb) mimic TSH, causing diffuse hyperplasia and hormone overproduction; TSH is suppressed. Anti-TPO antibodies characterise Hashimoto thyroiditis; release of preformed hormone occurs in thyroiditis.