What is Vitamin D?

Vitamin D is a fat-soluble vitamin and hormone that your body produces when your skin is exposed to sunlight. The 25-hydroxyvitamin D (25-OH) blood test is the most accurate way to measure your vitamin D status. It plays a critical role in calcium absorption, bone health, and immune function.

Why is it Important?

Vitamin D deficiency is one of the most common nutritional deficiencies worldwide, affecting an estimated 1 billion people. Low levels are linked to weakened bones (osteoporosis), increased infection risk, fatigue, muscle weakness, and mood disorders. Adequate vitamin D supports bone strength, immune defense, and overall well-being.

Normal Ranges

Group	Range	Unit
Deficient	< 20	ng/mL
Insufficient	20 – 29	ng/mL
Sufficient	30 – 100	ng/mL
Potentially Toxic	> 100	ng/mL
Sufficient	75 – 250	nmol/L

What Do Abnormal Results Mean?

Low Vitamin D (Deficiency)

Common causes include:

Limited sun exposure (northern climates, indoor lifestyle)
Dark skin pigmentation (reduced UV absorption)
Obesity (vitamin D sequestered in fat tissue)
Malabsorption disorders (celiac disease, Crohn's)
Kidney or liver disease
Certain medications (anticonvulsants, glucocorticoids)

High Vitamin D (Toxicity)

Common causes include:

Excessive supplementation (usually > 10,000 IU/day)
Granulomatous diseases (sarcoidosis)
Some lymphomas

Vitamin D toxicity can cause dangerous calcium buildup (hypercalcemia), leading to nausea, kidney stones, and cardiac arrhythmias.

When Should You Get Tested?

Your doctor may order a vitamin D test if you have risk factors for deficiency — limited sun exposure, dark skin, obesity, osteoporosis, chronic fatigue, frequent infections, or if you are over 65. It is also recommended during pregnancy and for people with malabsorption conditions.