Regulating the production of vitamin D
Several factors strictly control the amount of active vitamin D produced in the kidneys and in other tissues. The biggest factor is the result of self-regulation. As the amount of calcitriol increases, it blocks the production of more calcitriol.
Another important substance that stimulates the production of calcitriol is the circulating amount of another hormone called parathyroid hormone. When blood calcium levels fall, parathyroid hormone levels increase and this promotes the conversion of 25(OH)D3 into calcitriol within the kidneys. Concentrations of calcium and phosphate in the blood also control the production of calcitriol by the kidneys even without parathyroid hormone. As the calcium and phosphate levels fall, they stimulate the production of calcitriol in the kidneys.
The production of active vitamin D in organs and tissues other than the kidneys normally doesn’t spill over into the bloodstream to raise the active vitamin D in the blood. For example, during pregnancy the placenta makes calcitriol but at best a negligible amount enters the maternal circulation (pregnant women without kidneys have very low calcitriol levels despite the placenta making calcitriol). However, in certain diseases, such as sarcoidosis (a disease where swelling occurs in the lymph nodes, lungs, liver, skin, and other tissues), immune cells called macrophages produce so much calcitriol that it spills over into the bloodstream and causes increased calcium in the blood.