What is the most common vitamin deficiency in the US?

Vitamin D is the most common vitamin deficiency in the United States — affecting over 40% of American adults. Deficiency is linked to fatigue, depression, poor immune function, bone loss, and muscle weakness. A Vitamin D (25-Hydroxy) blood test is the only way to confirm your actual level.

What vitamins should I test if I am always tired?

The most important vitamins to test for unexplained fatigue are Vitamin D, Vitamin B12, Folate, Magnesium, Iron, and Ferritin. The Basic Vitamin Panel or Comprehensive Vitamin Profile covers all of these in a single blood draw without requiring a doctor's visit.

What is the difference between serum magnesium and RBC magnesium?

Serum magnesium measures magnesium in the liquid part of your blood — but only about 1% of your body's magnesium is in the blood. RBC magnesium measures magnesium inside your red blood cells, which is a much more accurate reflection of your body's actual magnesium stores. Standard blood panels almost always use serum magnesium, which frequently misses true deficiency.

Should I test vitamins before starting supplements?

Yes. Testing before supplementing is the clinically correct approach. Taking high-dose Vitamin D without knowing your baseline level risks toxicity. Testing first gives you a precise starting point, helps you dose correctly, and lets you retest in 3 months to confirm your levels have improved.

Can I order vitamin blood tests without a doctor?

Yes. Personalabs provides the physician-signed lab order as part of your purchase. Order online, visit any Labcorp patient service center near you, and get results in your secure account within 24 to 48 hours. No referral, no insurance, no doctor visit required.

Vitamin & Nutrition Blood Tests Online

Vitamin D 25-Hydroxy Blood Test

📅 Last Updated: March 2026 | 🩺 Medically Reviewed by: Edward Salko, D.O. | 🧪 Lab Partner: Labcorp Also Known…

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