Calcium levels are regulated by two systems: parathyroid hormone (PTH) released by the parathyroid gland (green arrows and white text) and vitamin D (white arrows and yellow text). Low calcium levels stimulate the four parathyroid glands to secrete PTH. PTH acts on bones, intestines and kidneys. In bone, PTH induces calcium release by stimulation of bone resorption. It also stimulates the uptake of calcium and vitamin D from the intestine. Furthermore, it promotes reabsorption of calcium from the glomerular filtrate in the kidneys.
Vitamin D is absorbed from the intestine, converted into 25-OH vitamin D3 in the liver and into 1,25(OH)2vitamin D3 in the kidney. The 1,25(OH)2vitamin D3 is the active vitamin D, which enhances calcium uptake from the intestine and calcium release from bones. High calcium levels have a negative feedback effect on the secretion of PTH from the parathyroid glands. So, via actions of PTH and vitamin D derivatives, normal plasma calcium levels can be maintained.
I. Plasma 25-OH vitamin D3 levels are a reflection of dietary vitamin D intake.
II. Too much sunlight causes hypercalcemia.
Extra info: Vitamin D from the diet is converted into 25-OH vitamin D3 in the liver. Sunlight on the skin stimulates previtamin D production. This prehormone is spontaneously converted into vitamin D after 3 days. However, continuous sunlight causes also photodegradation of previtamin D and thus controls vitamin D production.