Parathyroid Hormones–Chemistry, Synthesis, Secretion, Functions and Regulations.

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 Parathyroid hormone (PTH), produced by the parathyroid glands, plays a crucial role in regulating calcium and phosphate homeostasis in the body. Here's an overview of parathyroid hormone, including its chemistry, synthesis, secretion, functions, and regulation:

1. Chemistry:

  • Parathyroid hormone is a small peptide hormone composed of 84 amino acids.

2. Synthesis:

  • Parathyroid hormone is synthesized and produced by the parathyroid glands, which are four small glands located on the posterior surface of the thyroid gland.
  • The synthesis of PTH is influenced by the calcium levels in the blood. When blood calcium levels drop below a certain threshold, it triggers the secretion of PTH.
  • PTH is synthesized as a precursor molecule called preproparathyroid hormone, which is cleaved to form proparathyroid hormone. Further cleavage of proparathyroid hormone results in the active hormone, PTH.

3. Secretion:

  • The secretion of parathyroid hormone is primarily regulated by the negative feedback mechanism involving blood calcium levels.
  • When blood calcium levels decrease, specialized calcium-sensing receptors on the surface of the parathyroid cells detect the drop in calcium concentration and stimulate the release of PTH.
  • PTH is released into the bloodstream and exerts its effects on target organs, primarily the bones, kidneys, and intestines.

4. Functions:

  • Parathyroid hormone plays several critical roles in calcium and phosphate homeostasis:
    • Bone: PTH stimulates the release of calcium from bones by activating osteoclasts, cells responsible for breaking down bone tissue.
    • Kidneys: PTH increases calcium reabsorption by the renal tubules, reducing calcium excretion in the urine. It also enhances the conversion of vitamin D to its active form, calcitriol, which, in turn, promotes intestinal calcium absorption.
    • Intestines: PTH indirectly increases calcium absorption in the intestines by stimulating the production of calcitriol, which enhances calcium uptake from the gut.

5. Regulation:

  • Parathyroid hormone secretion is primarily regulated by the concentration of ionized calcium (Ca2+) in the bloodstream.
  • When blood calcium levels fall, PTH secretion increases to mobilize calcium from bone stores, enhance renal calcium reabsorption, and stimulate calcitriol production.
  • Rising blood calcium levels inhibit PTH release via negative feedback.

In summary, parathyroid hormone (PTH) is a critical hormone that regulates calcium and phosphate levels in the body. It acts on the bones, kidneys, and intestines to maintain calcium homeostasis. PTH secretion is tightly regulated by the negative feedback system based on blood calcium levels, ensuring that calcium levels are maintained within a narrow range for proper physiological function. Dysregulation of PTH can lead to disorders such as hyperparathyroidism or hypoparathyroidism, with associated disturbances in calcium and phosphate metabolism.

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