Question

Compare and contrast the effects of ParaThyroid Hormone (PTH), Calcitriol (vit D3) and Calcitonin on calcium metabolism, phosphate metabolism, and bone growth. What other hormones are involved in bone growth, what bone cells are affected, and how does each type affect bone structure and growth?

Answer 1

Maintaining normal blood calcium and phosphorus concentrations is managed by cordinated effect of three hormones, parathyroid hormone, which acts to increase calcium ions, calcitrol, which acts to increase calcium ions, and calcitonin, which acts to decrease calcium ions, PTH serves to facilitates mobilization of phosphate from bone. to prevent detrimental increases in phosphate, parathyroid hormone also has a potent effect on the kidney to eliminate phosphate known as phosphaturic effect

When the blood calcium levels are low, the parathyroid glands produce and secrete PTH, this  causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH, also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium and magnesium in the kidney tubules from the urine filtrate. Pth also activates calcitrol from the kidneys,calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH. On the other hand when calcium concentration is very high, parafollicular cells of the thyroid gland secrete calcitonin, this hormone inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.

Other hormone involved in bone growth are the growth hormones, which increases length of long bones, enhances mineralization, and improves bone density.

Thyroxine which stimulates bone growth and promotes synthesis of bone matrix