Parathyroid Hormone

Parathyroid hormone (PTH) is secreted by the parathyroid glands, which adhere to the posterior surface of the thyroid gland (see fig. 17.9). These glands release PTH when the blood calcium is too low. A mere 1% drop in the blood calcium level doubles the secretion of PTH.

Saladin: Anatomy & 7. Bone Tissue Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 7 Bone Tissue 233

Chapter 7 Bone Tissue 233

Calcitriol Synthesis

Figure 7.15 Calcitriol Synthesis and Action. Starting at the upper left, ultraviolet rays stimulate epidermal keratinocytes to convert 7-dehydrocholesterol into vitamin D3. The liver adds an —OH group to vitamin D3, which converts it to calcidiol; the kidney adds another, which converts this to calcitriol, the most potent form of vitamin D. Calcitriol acts on the bones, kidneys, and small intestine to raise blood calcium and phosphate levels and promote bone deposition.

Figure 7.15 Calcitriol Synthesis and Action. Starting at the upper left, ultraviolet rays stimulate epidermal keratinocytes to convert 7-dehydrocholesterol into vitamin D3. The liver adds an —OH group to vitamin D3, which converts it to calcidiol; the kidney adds another, which converts this to calcitriol, the most potent form of vitamin D. Calcitriol acts on the bones, kidneys, and small intestine to raise blood calcium and phosphate levels and promote bone deposition.

PTH raises the blood calcium level by four mechanisms (fig. 7.16b):

  1. PTH binds to receptors on the osteoblasts, stimulates them to secrete osteoclast-stimulating factor, and this in turn raises the osteoclast population and promotes bone resorption.
  2. PTH promotes calcium reabsorption by the kidneys.
  3. PTH promotes the final step of calcitriol synthesis in the kidneys, thus enhancing the calcium-raising effect of calcitriol.
  4. PTH inhibits collagen synthesis by osteoblasts, thus inhibiting bone deposition.

Notwithstanding these normal effects of PTH, the intermittent secretion (or injection) of PTH at low levels can cause bone deposition. PTH can therefore increase or decrease bone mass, depending on other factors such as exercise, stress on the bones, calcium and phosphate availability, and the action of vitamin D and other hormones.

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