Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions.

Bone-specific alkaline phosphatase (BAP) is synthesized by the osteoblasts and is presumed to be involved in the calcification of bone matrix, though its precise role in the formation process is unknown. In the serum of most healthy individuals, bone and liver isoenzymes of the tissue non-specific AP gene predominate in approximately equal proportions. ALP isoenzymes mainly exist in bones, liver, intestine, placenta, mammary glands and kidneys.

Fibroblast growth factor 23 (FGF23) has become the third key player of phosphate/calcium metabolism after PTH and vitamin D. FGF23 is a protein synthesized by osteocytes that acts mainly as a phosphaturic factor and a suppressor of 1α hydroxylase activity in the kidney.

Parathyroid hormone is a peptide hormone secreted by the parathyroid glands, which are located adjacent to the thyroid gland. Parathyroid hormone alters serum calcium via actions on three target organs: bone, intestinal mucosa, and kidney.