What Is Vitamin D Therapy and How Does It Work?

Vitamin D is a fat-soluble vitamin that also functions as a hormone. Its best-known role is regulating calcium and phosphorus absorption, which is important for building and maintaining strong bones. Beyond the skeletal system, it supports immune function, muscle movement, and nerve communication. Vitamin D therapy is a medical treatment that corrects a deficiency in this nutrient, with the goal of restoring levels to a sufficient range.

Understanding Vitamin D Deficiency and the Need for Therapy

A diagnosis of vitamin D deficiency is determined through a blood test measuring 25-hydroxyvitamin D, or 25(OH)D. Levels below 20 nanograms per milliliter (ng/mL) are considered deficient, and levels between 21 and 29 ng/mL are classified as insufficient. Therapy is considered when these levels are low, especially if a patient has symptoms or related health conditions.

Insufficient sunlight exposure is a main cause, as the skin produces vitamin D from ultraviolet-B (UVB) rays. This risk is higher for people in northern latitudes, those who spend most of their time indoors, or individuals who consistently wear sunscreen. Other risk factors include older age, as the skin’s synthesis ability diminishes, and darker skin pigmentation, which reduces vitamin D production.

Certain medical conditions also contribute to low levels. Malabsorption syndromes like Crohn’s disease impair the intestine’s ability to absorb the vitamin. Obesity is another factor, as vitamin D can become trapped in fat tissue, making it less available. The symptoms of a deficiency can be subtle and may include:

• Fatigue
• Bone pain
• Muscle weakness
• Mood changes

Types and Delivery Methods of Vitamin D

Vitamin D therapy uses two main forms: ergocalciferol (D2) and cholecalciferol (D3). Vitamin D2 is derived from plant sources like fungi, while D3 is produced by animals, including humans, from sunlight exposure and is found in foods like fatty fish. Both forms are effective at raising blood levels of vitamin D because they are converted in the liver to 25-hydroxyvitamin D.

Some studies suggest D3 may be more effective at raising and maintaining vitamin D concentrations for longer periods. For this reason, many healthcare providers recommend D3 supplements. The choice can also depend on a patient’s dietary preferences, as those on a vegan diet may prefer the plant-derived D2.

The most common delivery method is oral supplements, which are available as tablets, capsules, chewables, and liquids. In cases of severe deficiency or malabsorption, vitamin D can be administered by intramuscular injection. This method bypasses the digestive system and delivers the vitamin directly into the bloodstream.

Implementing Vitamin D Therapy: Dosages and Monitoring

Vitamin D therapy dosages are individualized by a healthcare professional based on the patient’s baseline vitamin D level, body weight, age, and therapeutic goal. Self-prescribing high doses without medical supervision is strongly discouraged. Treatment often begins with a high “loading” dose to rapidly increase the body’s stores, followed by a lower “maintenance” dose to keep levels in the target range.

Regular monitoring is a component of safe and effective therapy. Periodic blood tests measure serum 25-hydroxyvitamin D levels to ensure they are rising and being maintained within the desired range, allowing a provider to make dosage adjustments. Serum calcium levels may also be monitored, as excessive vitamin D can lead to high blood calcium, or hypercalcemia.

Although rare, vitamin D toxicity can cause nausea, vomiting, frequent urination, and confusion. Adherence to the prescribed dose and routine follow-up appointments are necessary to maximize benefits while minimizing risks.

Therapeutic Applications of Vitamin D

Vitamin D therapy is an established treatment for bone disorders caused by severe deficiency. In children, it is used to prevent and treat rickets, a condition of soft, weak bones. In adults, the equivalent condition is osteomalacia, which is also treated with vitamin D to restore bone mineralization.

The therapy is also part of managing osteoporosis, a disease that makes bones weak and brittle. Prescribed with calcium and other medications, vitamin D ensures adequate calcium absorption. This helps slow bone mineral loss and reduce fracture risk, particularly in older adults.

Beyond bone health, research has explored vitamin D’s supportive role in managing autoimmune diseases and infections. While sufficient vitamin D is important for immune function, its specific therapeutic use for these conditions is still under study. The most validated applications remain correcting deficiency and treating related bone diseases.