The Vitamin D test is a blood test that measures the level of 25-hydroxyvitamin D in the body to assess the status of this vitamin, which plays a vital role in calcium absorption, bone health, immune system function, and inflammation regulation. This test is commonly used to diagnose vitamin D deficiency or toxicity. Deficiency (levels below 20 nanograms per milliliter) is associated with symptoms such as weak bones, muscle pain, fatigue, and an increased risk of osteoporosis, rickets in children, or osteomalacia in adults. Levels above 100 nanograms per milliliter can also be toxic and dangerous. Factors contributing to vitamin D deficiency include lack of sunlight, poor diet, malabsorption disorders (such as celiac disease), kidney or liver diseases. This test is useful in monitoring treatment with vitamin D supplements, evaluating patients with metabolic bone disorders, or individuals with non-specific symptoms such as muscle weakness. Interpretation of results is done alongside clinical symptoms and other tests.