Parathyroid Hormone Physiology

Overview

Parathyroid Hormone (PTH) is a peptide hormone produced and secreted by the parathyroid chief cells of the parathyroid glands. In general, secretion of PTH serves to increase blood calcium and decrease phosphate]] concentrations.

Regulation

PTH secretion by parathyroid chief cells is highly sensitive to and varies inversely with blood calcium concentration
When calcium concentration is high, PTH secretion is low
When calcium concentration is low, PTH secretion is high

PTH Actions

The primary loci of PTH action are the bone and the kidneys although indirectly PTH can modulate the GI System. In general, PTH serves to resorb calcium and phosphate from the bone, increase calcium resorption from the alimentary tract, and increase phosphate renal excretion. Consequently, the net effect of higher PTH levels is an increase in blood calcium and a decrease in phosphate.
It should be pointed out that because phosphate chemically binds calcium, as described in Calcium and Phosphate Physiologic Forms, the reduction in phosphate levels serves to liberate additional ionized calcium (Ca⁺⁺) which is the actual physiologically active form of the atom

Recall that a small fraction of the bone's calcium and phosphate is deposited in the form of readily exchangeable bone salts. PTH acts on osteocytes throughout the body's bones to rapidly leech out those salts and excrete them into the extracellular fluid, thus rapidly increasing extracellular calcium and phosphate levels within minutes.

The vast majority of calcium and phosphate in the body is deposited in the bones as a highly stable crystalline solid known as hydroxyapatite which requires extended periods of time to digest. PTH activates existing osteoclasts and promotes development of new osteoclasts to resorb hydroxyapatite and excrete the liberated calcium and phosphate into the extracellular fluid. This process occurs on the order of days to months and if continued over long periods of time this can lead to significant bone weakness.

As discussed in Regulation of Calcium Excretion PTH acts on the late distal tubule and collecting ducts to enhance resorption of tubular calcium. In addition, as discussed in Regulation of Phosphate Excretion, PTH reduces resorption of tubular phosphate causing increased urinary excretion of phosphate. Finally, as discussed in Vitamin D Physiology, PTH induces expression of a key renal enzyme critical for Vitamin D biosynthesis; consequently, higher PTH levels in turn result in higher Vitamin D levels.

As discussed in Vitamin D Physiology, Vitamin D enhances alimentary absorption of calcium and phosphate; consequently, because PTH controls Vitamin D biosynthesis, PTH can be thought to indirectly enhance GI absorption of calcium and phosphate.

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