Examinations
Examinations
The 1,25-Dihydroxy Vitamin D test measures the biologically active form of Vitamin D, which is synthesized in the liver and kidneys. It is the most potent metabolite of Vitamin D and plays a crucial role in calcium and phosphorus metabolism.
Unlike the 25-Hydroxy Vitamin D test, this test is not commonly used for assessing Vitamin D deficiency because it may show normal results even in individuals with overall Vitamin D deficiency. Instead, it is typically used for evaluating calcium-related disorders and monitoring kidney function in patients with renal osteodystrophy or chronic kidney disease.
Vitamin D Metabolism
- Liver: Vitamin D is converted into 25-Hydroxy Vitamin D (25-OH D), a storage form with moderate biological activity.
- Kidneys: 25-OH D is further converted into 1,25-Dihydroxy Vitamin D (1,25-(OH)₂D or Calcitriol), the most active form responsible for regulating calcium and phosphorus homeostasis.
- Sample Type: Venous blood
- Fasting Required: 8–14 hours (minimum 8 hours)
- Avoid: Food, alcohol, and excessive supplementation before the test
- Parathyroid gland disorders
- Kidney failure
- Primary hyperparathyroidism
- Hypoparathyroidism
- Pseudohypoparathyroidism
- Renal osteodystrophy
Increased 1,25-Dihydroxy Vitamin D Levels
- Excess parathyroid hormone (PTH)
- Sarcoidosis
- Certain lymphomas (which can produce 1,25-(OH)₂D outside the kidneys)
- Hypercalcemia due to excessive Vitamin D activation
Decreased 1,25-Dihydroxy Vitamin D Levels
- Kidney disease (one of the earliest changes in renal failure)
- Chronic kidney disease
- Vitamin D deficiency-related hypocalcemia
- Hypoparathyroidism
- Pseudohypoparathyroidism
- Severe calcium and phosphate metabolism disorders
Important Considerations
This test is not recommended for diagnosing Vitamin D deficiency or monitoring Vitamin D supplementation in most patients. Instead, 25-Hydroxy Vitamin D (25-OH D) is the preferred test for those purposes.