Parathyroid gland

Parathyroid glands in an embryo of approx. 4 weeks. The glands have not yet migrated to their final position from the third and fourth pharynx bag.

The parathyroid glands (from para- and thyroid) known as the Owen-Sandström-Gley glands are endocrine glands located in the the neck, behind the thyroid lobes. They produce parathyroid hormone or parathormone (PTH). There are usually four parathyroid glands, two upper and two lower. When there is an additional gland, it is usually located in the mediastinum, in relation to the isthmus, or within the thyroid gland.

Anatomy

The parathyroid gland is lentil-shaped, measuring approximately 5x3x3 mm and weighing 30 mg each. Its color is variable between yellow, reddish or brownish tones and it has a soft consistency. The inferior parathyroid glands lie in close relationship with the inferior thyroid artery and the recurrent laryngeal nerve. On the other hand, the superior glands are in relation to the superior thyroid artery. It is irrigated by voluminous arteries, with respect to its size, which is why it bleeds very easily in surgical procedures. The superior parathyroid receives an arterial branch from the superior thyroid artery, and the inferior parathyroid from the inferior thyroid artery.

Lymphatics drain with the thyroid glands into the deep cervical and paratracheal nodes.

Histologically they are surrounded by a capsule and are made up of three types of cells, the main cells responsible for the production of parathyroid hormone (PTH), the oxyphil cells and the aqueous cells whose function is unknown. Parathyroid hormone participates in the control of calcium and phosphorus homeostasis, as well as in bone physiology.

Parathyroid hormone or parathormone

It is secreted by the principal cells of the parathyroid gland, it is a polypeptide of 84 amino acids whose molecular weight is approximately 9500 Da. Among its functions are the following:

• It facilitates the absorption of calcium, vitamin D (in its natural form), and phosphate; together in the intestine.
• It increases the calcium resorption of the bones, by producing more osteoclasts from the mesenchymatous stem cells of the bone marrow, delaying the conversion of these into osteoblasts. Osteoclastes absorb the bone by release of protein hormones released by lisosomes, and secretion of several acids including citric acid and lactic acid.
• It reduces the renal excretion of calcium and increases the renal excretion of phosphate, causing urinary excretion in greater concentration.
• It increases the resorption of calcium in the intestine. Induces an increase in the formation of 1.25-dihydroxyxycalciferol (active form of vitamin D, calcitriol or vitamin D3) from 25-hydroxyxycholecalciferol (calciphediol) in the kidneys, vitamin D3 then acts at the level of the intestinal epithelium by increasing the absorption of calcium, thus increasing the levels of plasma calcium (normal value of calcium: 9.4 mg). Then due to a negative feedback or feedback mechanism, high levels of plasma calcium inhibit the secretion of PTH as well as helping to reabsorb calcium through the kidneys.
• In summary: regulates the amount of calcium presented in the blood.

Hyperparathyroidism and Hypoparathyroidism

Excess calcium function of the parathyroid glands is known as hyperparathyroidism, and it usually occurs with elevated plasma calcium levels and bone fragility, which conditions a greater susceptibility to suffering fractures. Insufficient function of the parathyroid glands (hypoparathyroidism) is much less common, and usually occurs after surgery on the thyroid gland, leading to hypocalcemia.
There are some much rarer diseases, which appear to be due to alterations in the parathyroid hormone receptor, such as Jansen's metaphyseal chondroplasia and Blomstrand's chondroplasia.