Calcium-sensing membrane receptors in the parathyroid gland monitor calcium levels in the extracellular fluid. Low levels of calcium stimulates the release of parathyroid hormone (PTH) from chief cells of the parathyroid gland .  In addition to its effects on kidney and intestine, PTH increases the number and activity of osteoclasts. The increase in activity of already existing osteoclasts is the initial effect of PTH, and begins in minutes and increases over a few hours.  Continued elevation of PTH levels increases the abundance of osteoclasts. This leads to a greater resorption of calcium and phosphate ions. 
Help I have a friend who is a gym goer Im not sure of his quantity or how long he has been taking steroids, but stopped recently because he had really bad neck pain. No dr or scan, ultrasound etc showed anything. Put on huge pain killer amounts didnt help alot but felt after about six weeks some relief. Until today when he thinks a prior knee issue has flared up. If this a result of steroid abuse how long before it heals? Im pretty sure he wont touch them again. He can handle all over aches and pains but these last two injuries have had him off work.
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