Parathyroid hormone (PTH)

General Information

The parathyroid glands secrete a polypeptide hormone - the pre-pro-parathyroid hormone, which consists of 115 amino acid residues. Upon entering the endoplasmic reticulum, it loses 25 amino acids and is converted to pro-parathyroid hormone (90 amino acids), then As a parathyroid hormone (PTH). PTH contains 84 amino acids. In the liver, kidneys, bones, and parathyroid glands, PTH (1-84) is metabolized to form C-terminal, N-terminal, and median fragments. PTH and the N-terminal fragment have hormonal activity.

The main regulator of PTH secretion is the extracellular calcium level on the principle of negative feedback. Parathyroid hormone, calcitonin, vitamin D are the main mediators of calcium homeostasis. The target of these hormones are bone tissue, kidneys and small intestine. Decreased blood calcium levels increase the secretion of PTH, which binds to specific receptors and has the following effects:

The effect of PTH on bone tissue.

It has both anabolic and catabolic effects on bone tissue. PTH receptors are found on osteoblasts and osteocytes, but not on osteoclasts.

With increased levels of PTH, osteoclasts become activated and bone resorption is enhanced. This effect of PTH is mediated by osteoblasts, which are produced by various mediators that have a strong stimulatory effect on osteoclast differentiation and proliferation. At consistently high concentrations of PTH, bone resorption outweighs its production, leading to osteopenia.

The action of PTH on the intestine.

Increases calcium absorption in the gastrointestinal tract indirectly by regulating vitamin D synthesis in the kidneys.

Action of parathyroid hormone on the kidneys:

- Stimulates calcium reabsorption in the distal tubules;

- Increases the excretion of phosphates;

- Increases the excretion of bicarbonate, which leads to alkalization of urine;

- Increase urinary clearance (increase urine volume);

- Stimulation of the synthesis of active forms of vitamin D in the proximal tubules.