Calcitriol is the active form of vitamin D. Parathyroid hormone (PTH) is secreted by the parathyroid glands in the neck. Both these chemicals are responsible for keeping the levels of calcium and phosphorous normal and also play a role in bone health. They can be measured in the blood. In some disorders, PTH or calcitriol levels are elevated, which results in increased calcium absorption from the gut, increased calcium in the bloodstream, and increased calcium excretion in the urine. Increased calcium in the urine can lead to the development of kidney stones. Increased calcium in the bloodstream can have adverse effects on the heart, gut, kidneys, and bones.