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Low 25-hydroxy vitamin D levels indicate a deficiency in vitamin D. Vitamin D deficiency can result from lack of exposure to sunlight, lack of adequate vitamin D in the diet, liver and kidney diseases, malabsorption, and medications such as phenytoin, phenobarbital, and rifampin. Vitamin D deficiency may lead to low blood calcium levels (hypocalcemia), thin or weak bones (osteoporosis and osteomalacia), and high levels of parathyroid hormone (secondary hyperparathyroidism).
High 25-hydroxy vitamin D indicates excess vitamin D (hypervitaminosis D) and is usually caused by too much vitamin D in the form of doctor-prescribed dietary supplements. This can lead to high calcium levels (hypercalcemia).
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