25-Hydroxy Vitamin D

25-Hydroxy Vitamin D [25(OH)D] is the main circulating form of vitamin D and the best laboratory indicator of vitamin D status in the body. It reflects vitamin D produced from sun exposure and obtained from food and supplements.

Why 25(OH)D Is Measured (Not 1,25-Dihydroxy Vitamin D)

• Longer half-life (≈2–3 weeks) → stable indicator
• Represents total body vitamin D stores
• 1,25-dihydroxy vitamin D (active form) can be normal or elevated even in deficiency

1️⃣ Bone Health & Calcium Regulation

25(OH)D is essential for:

• Calcium and phosphate absorption in intestines
• Bone mineralization
• Parathyroid hormone (PTH) regulation

Deficiency can cause:

• Rickets (children)
• Osteomalacia (adults)
• Osteoporosis
• Increased fracture risk

2️⃣ Muscle Function

Low levels are associated with:

• Muscle weakness
• Increased fall risk (especially in elderly)

3️⃣ Immune System Function

Vitamin D plays a role in:

• Innate immunity
• Modulating inflammation
• Reducing susceptibility to infections
• Low levels have been associated with:
• Increased respiratory infections
• Possible autoimmune disorders (association, not definitive causation)

4️⃣ Chronic Disease Associations

Research links low 25(OH)D levels with increased risk of:

• Cardiovascular disease
• Type 2 diabetes
• Certain cancers
• Multiple sclerosis

(These are associations; causation is not fully established.)

5️⃣ Vitamin D Toxicity (Hypervitaminosis D)

Usually due to excessive supplementation.

Symptoms include:

• Hypercalcemia
• Nausea/vomiting
• Polyuria
• Kidney stones
• Confusion

When Is 25(OH)D Testing Indicated?

Testing is recommended in:

• Osteoporosis or fracture history
• Chronic kidney or liver disease
• Malabsorption syndromes
• Hyperparathyroidism
• Long-term steroid use
• Limited sun exposure
• Obesity
• Darker skin pigmentation
• Routine screening of healthy individuals is debated.