Structure and position
Each parathyroid gland is a flattened oval body about the size of a lentil, roughly 6 mm long, and yellowish-brown. The colour and a distinct hilum with a small vessel help surgeons tell a parathyroid from fat, lymph nodes and thyroid nodules, which it closely resembles.
The glands lie on the posterior surface of the thyroid lobes, usually outside the true fibrous capsule of the thyroid but inside the fascial sheath formed by the pretracheal fascia. Most people have four glands. A minority have five or more, and some have fewer than four detectable glands.
Superior parathyroid glands
The superior glands are the more constant pair. They usually lie on the back of the upper half of each lobe, at about the level of the cricoid cartilage, above the point where the inferior thyroid artery crosses the recurrent laryngeal nerve. They lie posterior (dorsal) to the plane of the nerve.
Inferior parathyroid glands
The inferior glands usually lie near the lower pole of the thyroid, below the inferior thyroid artery and anterior (ventral) to the recurrent laryngeal nerve. Their position is more variable, from the angle of the mandible to the superior mediastinum. Common ectopic sites are the thyrothymic ligament and the upper thymus.
Development
The parathyroid glands develop from the endoderm of the third and fourth pharyngeal pouches, and their final positions are reversed. The inferior glands arise from the third pouch together with the thymus. As the thymus descends into the thorax it pulls these glands further down, past the glands from the fourth pouch, which become the superior parathyroids.
This long migration explains why the inferior glands are the ones most often found in unusual places: along the path of thymic descent, within the thymus, inside the carotid sheath or, rarely, within the thyroid itself. The superior glands travel a short distance and stay close to the upper thyroid.
Failure of third and fourth pouch development, as in 22q11.2 deletion (DiGeorge) syndrome, causes absent parathyroids and thymus, with neonatal hypocalcaemia and T-cell immunodeficiency.
Relations
The recurrent laryngeal nerve and the inferior thyroid artery are the key relations of the parathyroid glands. The nerve ascends in or near the tracheo-oesophageal groove and passes close to both glands on each side, so it is at risk in any parathyroid operation.
- Anteriorly: the thyroid lobe.
- Posteriorly: the oesophagus, and the pharynx for the superior glands.
- Medially: the trachea, larynx and recurrent laryngeal nerve.
- Laterally: the carotid sheath.
Enlarged superior glands tend to sink downwards and backwards behind the oesophagus into the posterior mediastinum. Enlarged inferior glands tend to move forwards into the anterior mediastinum along the thymus.
Blood supply and innervation
The inferior thyroid artery, a branch of the thyrocervical trunk, supplies both parathyroid glands on each side in most people. The superior gland can also be supplied by the superior thyroid artery or by an anastomosis between the two thyroid arteries along the back of the lobe. Each gland has a single small end artery, which is why careful dissection close to the thyroid, preserving these branches, is needed to keep the glands alive.
Venous blood drains into the thyroid venous plexus. Lymph drains with that of the thyroid to the deep cervical and paratracheal nodes. Sympathetic fibres from the cervical ganglia are vasomotor; secretion of parathyroid hormone is controlled directly by the ionised calcium concentration, sensed by calcium-sensing receptors on the chief cells.
Function
Parathyroid hormone raises serum calcium by increasing osteoclastic bone resorption, increasing calcium reabsorption in the distal renal tubule, promoting renal phosphate loss, and stimulating renal 1-alpha-hydroxylation of vitamin D, which increases calcium absorption from the gut.
Clinical relevance
Hypocalcaemia is the commonest significant complication of total thyroidectomy. Damage to the parathyroid blood supply or accidental removal of the glands causes perioral and fingertip tingling, cramps and tetany, usually within a few days. It is often temporary. Chvostek sign (facial twitching on tapping over the facial nerve) and Trousseau sign (carpal spasm with a blood pressure cuff inflated) are the bedside tests. A devascularised gland can be minced and autotransplanted into sternocleidomastoid.
Primary hyperparathyroidism is most often caused by a single adenoma. Raised calcium causes renal stones, bone pain, abdominal pain, constipation and low mood. Localisation with ultrasound and sestamibi scanning allows a targeted, minimally invasive excision; knowledge of the ectopic sites matters when the adenoma is not where expected.