The Vitamin D, 25-Hydroxy test measures levels of this crucial vitamin in the blood. Deficiency in vitamin D can lead to bone pain, muscle weakness, and an increased risk of fractures. It may also affect immune function and overall health. Excessive vitamin D levels can cause toxicity, leading to symptoms such as nausea, vomiting, and kidney problems. This test is vital for diagnosing vitamin D deficiencies and managing vitamin D intake.