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PTH
PTH Full Name
parathyroid hormone
PTH Introduction
Parathyroid hormone (PTH) is a polypeptide that is synthesized and cleaved into an active form within the parathyroid glands, which are small endocrine glands located in the neck behind the thyroid gland. PTH is initially synthesized as a large precursor molecule called pre-pro-PTH, consisting of 115 amino acids. It undergoes two cleavage steps to form active PTH, which is composed of 84 amino acids. This active form is responsible for the hormone's storage, secretion, and functions in the body. The entire process of synthesis, cleavage, and storage takes less than an hour. When blood calcium levels are low, PTH secretion can occur rapidly, within a few seconds. PTH is released through exocytosis, where it is carried in membrane vesicles to the cell membrane and released when the vesicles fuse with the outer membrane. The active PTH has a short half-life of a few minutes and is quickly cleared from the bloodstream by the kidneys and liver.
PTH has several effects on various organ systems in the body. In the bones, PTH stimulates the activity of osteoclasts, cells responsible for bone resorption. This process releases calcium from the bones into the bloodstream, increasing circulating calcium levels. PTH also indirectly stimulates osteoblasts, cells involved in bone formation, by increasing the production of factors that promote osteoblast differentiation and function.
In the kidneys, PTH plays a role in calcium and phosphate regulation. PTH increases the reabsorption of calcium in the distal convoluted tubule of the kidneys, reducing the excretion of calcium in urine and increasing its retention in the body. Additionally, PTH inhibits the reabsorption of phosphate in the proximal tubules of the kidneys, leading to increased phosphate excretion. This indirect action of PTH on phosphate helps to decrease the formation of insoluble calcium-phosphate salts, which could precipitate and reduce the levels of ionized calcium in the blood.
Furthermore, PTH is involved in the activation of vitamin D. It stimulates the production of 1alpha-hydroxylase enzyme in the kidneys, which converts inactive vitamin D (25-hydroxycholecalciferol) into its active form (1,25-dihydroxycholecalciferol or calcitriol). Calcitriol enhances the absorption of calcium from the intestines, promoting its entry into the bloodstream.
Figure 1. Mechanisms of action of parathyroid hormone.
(Source: Casarim, A. L., 2020)
Overall, PTH plays a vital role in the regulation of calcium and phosphate levels in the body. It acts on the bones, kidneys, and intestines to ensure that adequate amounts of calcium are available for various physiological processes. Imbalances in PTH secretion or function can lead to disorders such as hyperparathyroidism (excessive PTH production) or hypoparathyroidism (insufficient PTH production), which can have significant effects on bone health and mineral metabolism.
Alternate Names for PTH
PTH; parathyroid hormone; PTH1; parathyrin; prepro-PTH; parathormone; parathyroid hormone 1; preproparathyroid hormone;
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