<unk>  is the active form of vitamin D, normally made in the kidney. It is also known as 1,25-dihydroxycholecalciferol. It is a hormone which binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes.<unk>  increases blood calcium (Ca2+) mainly by increasing the uptake of calcium from the intestines. It can be given as a medication for the treatment of low blood calcium and hyperparathyroidism due to kidney disease, low blood calcium due to hypoparathyroidism, osteoporosis, osteomalacia, and familial hypophosphatemia, and can be taken by mouth or by injection into a vein. Excessive amounts or intake can result in weakness, headache, nausea, constipation, urinary tract infections, and abdominal pain. Serious side effects may include high blood calcium and anaphylaxis. Regular blood tests are recommended after the medication is started and when the dose is changed.<unk>  was identified as the active form of vitamin D in 1971 and the drug was approved for medical use in the United States in 1978. It is available as a generic medication. In 2017, it was the 256th most commonly prescribed medication in the United States, with more than one million prescriptions. It is on the World Health Organization's List of Essential Medicines.
Calcitriol
Parathyroid hormone
Estriol
Estriol 
Parathyroid hormone-related protein
Cholecalciferol
Ergocalciferol
Calcipotriol