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Calcitriol is the active form of vitamin D. It is a substance that occurs naturally in the body, and is responsible for increasing levels of calcium in the blood. It is also a medication that is used to treat or prevent low calcium levels due to various diseases.
Chemically, calcitriol may be referred to as 1,25-dihydroxycholecalciferol or 1,25-(OH)2D3. When referring to the medication, calcitriol is the generic name. Available brand names include Rocaltrol®, Calcijex®, and Decostriol®.
Most vitamin D is naturally derived from sunlight exposure. The ultraviolet rays convert 7-dehydrocholesterol in the skin to cholecalciferol, which is an inactive form of vitamin D. The kidneys then convert cholecalciferol to 1,25-dihydroxycholecalciferol, which is active and able to perform normal physiologic functions. When levels of calcium in the blood decrease, the kidneys make more calcitriol.
Calcitriol increases levels of calcium in the blood in three different ways. The most important is increasing dietary calcium absorption from the small intestines. Calcitriol also enhances reabsorption of calcium in the kidneys, and in concert with parathyroid hormone (PTH), stimulates the release of calcium from the bones. All of these mechanisms assure that blood levels of calcium are maintained within a narrow range.
When used as a drug, calcitriol helps to treat conditions that cause low blood calcium levels. Chronic low levels of calcium can lead to the release of calcium from the bones, which in turn leads to weakened bones, and eventually osteoporosis. This is primarily a problem in patients with kidney disease who are receiving dialysis, and in patients with an impaired or nonfunctional parathyroid gland. The latter is called hyperparathyroidism, and may be due to surgical removal or an inherited condition.
In addition to treating low calcium levels, calcitriol is also used to treat patients with weakened bones caused by a lack of enough vitamin D. This is called osteomalacia in adults and rickets in children. Finally, calcitriol is used in combination with calcium supplements to prevent or treat osteoporosis that is the result of menopause or medications such as corticosteroids. For maximum effectiveness in these conditions, patients must have adequate calcium intake in their diet.
Calcitriol is available as a capsule, an oral liquid, and as an injection. It is generally very well tolerated. The primary side effect is too much calcium in the blood, called hypercalcemia, which can be prevented with close monitoring of blood calcium levels. Common signs of hypercalcemia include nausea and vomiting, constipation, drowsiness, decreased reflexes, weakness, increased urination, loss of appetite, and weight loss. To reduce the risk of hypercalcemia, patients should avoid taking other calcium-containing products, such as antacids.