Calcitriol: Function & Diseases

Calcitriol is a very potent secosteroid that resembles steroid hormones because of its structure. It is hydroxylated in a wide variety of tissues, but predominantly in the kidneys, and is sometimes prescribed as a medication.

What is calcitriol?

Unlike other vitamins, vitamin D can be produced in the body itself. Deficiency symptoms only occur when there is too little sunlight or due to metabolic disorders. Calcitriol is the name given to the active form of vitamin D3 that is responsible for calcium balance in the body. With the help of a vitamin D receptor, it is transported into the nucleus of the cell, where the complex associates with the DNA. Calcitriol thereby acts

  • Promotes sperm movement
  • Against psoriasis and hair loss
  • As protection against different types of cancer
  • Favorable on the immune system, as many infections can be fought off better
  • Against osteoporosis

Function, effect and tasks

Calcitriol plays an essential role in the calcium balance of the body. Acute vitamin D deficiency can lead to bone diseases such as osteomalacia or rickets. Calcitriol is important for the absorption of phosphate and calcium in the small intestine, where calcium is absorbed by so-called calcium channel proteins. This is followed by transport through the cell and release into the blood. Calcium absorption is thereby dependent on the calcitriol supply. Bones are another important target organ for calcitriol. Bone tissue is subject to constant breakdown and regeneration, with parathyroid hormone, calcitriol and the calcium blood level interacting. Calcitriol is essential for the regular breakdown and formation of bones, and it also contributes to the formation of osteocalcin. Calcitriol also has a very positive effect on the immune system. It contributes to the improvement of the defense against infections and protects against various autoimmune diseases such as circular hair loss or psoriasis. Furthermore, it protects against certain types of cancer and has an effect on blood pressure, muscles and the nervous system. Calcitriol is also indispensable for the release of thyroid hormones and for insulin secretion. Calcitriol does not have a regulating effect, but influences other regulatory mechanisms. A normal calcitriol value depends on age and is between 20 and 67 ng/l in adults. Especially in children and pregnant women, vitamin D3 levels are extremely important to ensure healthy growth.

Formation, occurrence, properties, and optimal levels

Calcitriol is formed from 7-dehydrocholesterol. In the process, the hormone passes through the skin, liver and kidney as part of its synthesis. Calciol (vitamin D3) is formed in the skin. It then passes through the blood to the liver, where it is bound to a vitamin D receptor. In the liver, calciol is converted to calcidiol, and in the kidney it is finally converted to calcitriol with the help of a second OH group. Calcitriol is excreted mainly via the bile, with the majority participating in a so-called enterohepatic cycle and then returning to the body. The individual intermediates have very different half-lives, with the half-life of calcitriol being around three to five hours. The intake of vitamin D via various foods plays only a relatively minor role, since in areas rich in sunlight the need for vitamin D is largely covered by its own synthesis. However, especially in winter and autumn, foods that have a high ergocalciferol or cholecalciferol content should be consumed. These include foods such as fish, eggs or avocados.

Diseases and disorders

When calcitriol concentrations are very low due to vitamin D deficiency, rickets occurs. If patients are then treated with vitamin D, calcitriol levels rise above normal. Rickets is very rare, so the calcidiol level must always be measured in addition to the calcitriol level. Another condition is osteomalacia, a bone softness that occurs in adulthood and is caused by a deficiency of calcitriol. Also in renal and liver diseases, such as chronic renal insufficiency or liver cirrhosis, vitamin D absorbed through the skin cannot be sufficiently hydroxylated to calcitriol.If the calcitriol concentration is too high, a condition known as sarcoidosis can occur. In this case, tissue nodules form in the lungs and those affected suffer from coughing and shortness of breath. Vitamin D3 levels also rise in malignant tumors and in an overactive parathyroid gland. An increase in calcitriol levels can also occur after a kidney transplant. Furthermore, hereditary defects also trigger an increased production of calcitriol. Other symptoms that indicate vitamin D deficiency include:

  • Deformation of the legs as well as the spine.
  • Jaw deformation and dental malocclusions
  • Muscle weakness
  • Tooth loss
  • Increased irritability and nervousness
  • Myopia
  • Muscle spasms in the area of the feet, hands and lips

Very high doses of vitamin D can also lead to death in rare cases. For this reason, all preparations with over 1000 I.U. vitamin D are subject to prescription. In case of overdose with vitamin D, the following symptoms occur:

However, vitamin D poisoning can only occur due to the intake of vitamin D supplements.