Blood & Laboratory Values

Parathyroid Hormone (Parathyrin) – Function & Diseases

Parathyroid hormone (parathyrin)

Parathyroid hormone or parathyrin is formed in the parathyroid glands . The hormone plays an important role in regulating the calcium and phosphate balance.

What is parathyroid hormone?

Parathyroid hormone (parathyrin, PTH) is a linear polypeptide hormone produced by the parathyroid glands (parathyroid glands, epithelial bodies) and consists of a total of 84 amino acids .

In interaction with vitamin D and its direct antagonist, calcitonin , which is produced in the thyroid gland , the hormone regulates the calcium and phosphate balance in the human body.

In a healthy state, the reference value is about 11 to 67 ng/l in the blood.

Production, Education & Manufacturing

Parathyroid hormone is produced and secreted (released) by the parathyroid glands. The parathyroid glands are four small, lentil -sized glands that are located in pairs on the left and right of the thyroid gland.

The peptide hormone is synthesized in the hormone-producing main cells of the epithelium and released directly into the blood (endocrine secretion) due to the lack of independent ducts. Here, the hormone is first formed as a precursor consisting of 115 amino acids (prepro-hormone) on the membrane-bound ribosomes. Ribosomes are RNA-rich particles on which protein synthesis takes place in the cells.

The amino-terminal sequence is then co-translationally cleaved, ie during the translation of mRNA into the amino acid sequence. Another precursor of 90 amino acids (pro-parathyroid hormone) is formed, which is processed in the Golgi apparatus (protein-modifying cell organelle) to the finished parathyroid hormone.

Function, effect & properties

Together with vitamin D (calcitriol) and the thyroid hormone calcitonin, parathyroid hormone regulates the calcium and phosphate levels in the blood. The level of calcium in the blood is determined with the help of specific receptors on the membrane of the parathyroid cells (so-called G-protein-coupled calcium receptors).

A fall in the blood calcium concentration stimulates the production and release of parathyroid hormone in the parathyroid glands, while an increased blood calcium inhibits its secretion (negative feedback). Accordingly, for example, hypocalcemia (low calcium levels) forms the stimulus for the release of parathyroid hormone. The direct and indirect effects of the hormone cause an increase in the unbound, free calcium concentration in the blood via a stimulation of the adenylate cyclase (enzyme) in the bones and kidneys .

The osteoclasts of the bones and the reabsorption of calcium in the kidneys (reduced excretion via the kidneys with urine) are directly stimulated. In addition, the phosphate concentration in the blood is reduced by increased excretion via the kidneys (inhibited reabsorption). In order to prevent demineralization of the bones, the vitamin D or calcitriol synthesis is stimulated in parallel via the phosphate level that has fallen in this way ( hypophosphatemia ).

Calcitriol promotes bone remineralization by increasing calcium absorption in the small intestine . At the same time, the resulting increased calcium concentration in the blood inhibits the release of parathyroid hormone. Calcitonin fulfills an analogous function, which is released when the calcium level rises and promotes the incorporation of calcium in the bones while inhibiting osteoclast activity at the same time.

A permanent stimulation of the osteoclasts results in a gradual loss of bone mass. Therefore, for example, secondary hyperparathyroidism (overproduction of parathyroid hormone) is associated with osteoporosis in old age. A fragment of parathyroid hormone (made up of amino acids 1 to 34) is used therapeutically as a drug that stimulates bone formation.

Illnesses, Ailments & Disorders

Impairments of parathyroid hormone metabolism are generally divided into so-called hyperparathyroidism (overactive parathyroid glands) and hypoparathyroidism (underactive parathyroid glands). In hyperparathyroidism, parathyroid hormone is increasingly produced and secreted.

The hormone concentration in the blood is increased. If the hyperfunction can be traced back to an impairment of the parathyroid glands themselves, this is called primary hyperparathyroidism. This is usually caused by benign (hormone-producing parathyroid adenomas), in very rare cases by malignant tumors (parathyroid carcinomas).

In addition, an overactive parathyroid gland in connection with kidney, liver or intestinal diseases as well as a vitamin D or calcium deficiency (secondary hyperparathyroidism). A lack of vitamin D or calcium leads to a reduced blood calcium level, which in turn stimulates parathyroid hormone synthesis in the parathyroid glands. In the case of a long-term reduced calcium level, which can also manifest itself as a result of renal insufficiency (kidney weakness), for example, the parathyroid glands synthesize more and more parathyroid hormone.

In the long term, this overproduction can cause parathyroid hyperplasia (proliferation of parathyroid tissue), which in turn is associated with manifest, primary hyperparathyroidism. In the case of hypoparathyroidism, on the other hand, there is reduced production and release of the parathyroid hormone and the parathyroid hormone concentration in the blood is reduced. If the parathyroid glands do not react with an increased parathyroid hormone release despite a reduced calcium concentration, this can usually be attributed to a functional disorder of the parathyroid glands (primary hypoparathyroidism).

In many cases, primary hypoparathyroidism is caused by autoimmune diseases (including sarcoid ) or partial removal of tissue from the parathyroid glands (removal of the epithelial bodies or parathyroidectomy). In some cases, the parathyroid glands are also injured during thyroid surgery.

Progressive (advanced) tumors and an overactive thyroid gland can cause hypercalcaemia (permanently elevated calcium levels), which in turn is associated with a reduced concentration of parathyroid hormone. Likewise, an overdose of vitamin D leads to a reduced release of parathyroid hormone into the blood.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.