Parathyroid Anatomy

Fig.1: A rear view of the thyroid, showing the parathyroids lying on the back of the thyroid, close to the recurrent laryngeal nerveThe parathyroid glands are tan-coloured, bean-shaped structures, about the size of a grain of rice, weighing around 30-40 mg each. There are usually four of them, with two on each side, and they lie on the posterior surface of the thyroid gland.

They are symmetrically arranged in 80% of cases, with the two superior (upper) parathyroids lying about 1cm above the point where the inferior thyroid artery enters the thyroid. The two inferior (lower) parathyroids lie about 1cm below this point, usually around the lower pole of the thyroid.

There is quite a deal of variation in the number and location of glands:

  • 3% have less than 4 glands
  • 5-10% have more than four glands
  • 15-20% have ectopic glands (out of the usual locations)

Superior parathyroids are more rarely ectopic than inferior glands, having travelled a shorter distance during embryological life. However, superior parathyroids have been found as high as the level of the thyroid cartilage and even at the level of the hyoid bone, but usually if ectopic they are more likely to be retro-oesophageal or some other posterior location. Intrathyroidal parathyroids are more likely to be superior glands.

Inferior parathyroids are much more variable in position, although 60% will be around the lower pole of the thyroid. Ectopic locations can be anywhere from the angle of the mandible (jaw) all the way down into the chest as low as the pericardium (covering of the heart), including the carotid sheath. The most common ectopic location however, is in the thymus (or thyrothymic tract) in the upper chest, due to the codevelopment embryologically (see below). 

Fig.2: Relation of the parathyroids to the left recurrent laryngeal nerve, with superior glands behind and inferior glands in front of the plane of the RLNThe relative position of the parathyroids in relation to the recurrent laryngeal nerve is of more importance in surgery than their relation to the inferior thyroid artery (Fig. 2). Superior glands lie behind the plane of the nerve, while inferior glands lie in front of this plane, making the determination of which gland is which easier, and facilitating the finding of ‘lost’ glands during surgery.

The glands can be either within the fibrous capsule of the thyroid (intracapsular), or outside it (extracapsular). This assumes great importance in surgery as tumorous intracapsular parathyroids will expand locally within the thyroid capsule. Extracapsular parathyroids are not constrained and may expand downwards into the mediastinum either anteriorly or posteriorly.

Embryology

Parathyroid development begins around the fifth or sixth week of life at the level of the pharynx, with all parathyroids migrating down into the neck.

The superior parathyroids are derived from the fourth branchial pouches (along with the ultimobranchial bodies, which differentiate into the parafollicular or C-cells, that secrete calcitonin). Because the superior parathyroid glands migrate with the ultimobranchial bodies, they remain in contact with the posterior part of the middle third of the thyroid lobes, and may come to be found at the tip of the tubercle of Zuckerkandl.

The inferior parathyroids are derived from the third branchial pouches, along with the thymus. This embryological development of the inferior parathyroids in the same place as the thymus (which comes to lie below the inferior poles of the thyroid) explains why parathyroid tumours may be found within the substance of the thymus and in the mediastinum (Fig. 3).

So, the inferior glands actually start life above the superior glands, but descend below them. This longer process of descent by the inferior glands, gives great scope for misplacement, and explains why inferior parathyroids have more varied positions than superior parathyroids.


Fig.3: Specimen of the thymus gland containing a grey parathyroid tumour in its substanceBlood supply

In general, the majority of the blood supply to the parathyroids comes from branches of the inferior thyroid arteries.

Great variation exists however, with supply by the superior thyroid arteries, the thyroid ima artery, the laryngeal arteries, tracheal arteries or oesophageal arteries documented.

The superior glands get 80% supply from the inferior thyroid arteries, 15% from the superior and 5% from elsewhere. The inferior glands are 90% supplied by the inferior thyroid arteries and 10% from the superior thyroid arteries. Venous drainage is into the plexus of veins on the anterior surface (front) of the thyroid comprising the superior, middle and inferior thyroid veins.

Nerve supply

The parathyroids are supplied by thyroid branches of the cervical sympathetic ganglia with a mainly sensory function, detecting stretch within the glands that gives rise to the sensation of pain in some disorders.

Histology

Fig.4: Histology slide of normal parathyroid showing chief cells, and the larger oxyphil cellsThe glands are made up of four types of cell:

  • Chief cells - these are the cells which secrete the parathyroid hormone (PTH)
  • Oxyphil cells - these have a secretory function, and tend to become more common with age, but their precise role is not clear
  • Adipose tissue - these are fat cells which add bulk to the glands and increase with age and obesity
  • Fibrovascular stroma - this is fibrous tissue that gives form to the glands containing the capillaries supplying them with blood.

Under the microscope the parenchymal cells (the specialised chief and oxyphil cells) are arranged in thick branching cords throughout the stroma (the adipose and vascular tissue). Sheet and tubular formations are also seen (Fig. 4).