What is ferritin?
Ferritin is a protein that stores iron inside your cells. Think of it as your body's iron savings account. When your body needs iron — to carry oxygen in your blood, fuel your muscles, or support your brain — it withdraws from these ferritin stores.
A ferritin blood test tells you how much iron you have in reserve. And here's the important part: you can run low on ferritin (your savings) while your hemoglobin and iron levels (your checking account) still look "normal" on a standard blood test. This is why so many women are told their labs are fine — when they're actually depleted.
Why it matters for longevity
Low ferritin is the number one nutritional deficiency in women worldwide. It's incredibly common in women who menstruate, because monthly blood loss steadily depletes iron stores. Yet it often goes undiagnosed because many doctors only check hemoglobin, not ferritin.
When ferritin drops too low, the effects can be significant: crushing fatigue, hair loss, brain fog, poor exercise recovery, restless legs, difficulty concentrating, and feeling cold all the time. Many women spend years dealing with these symptoms before someone finally checks their ferritin.
Iron also plays a key role in thyroid function. Low ferritin can worsen or mimic thyroid problems, which is why your TSH (thyroid) and ferritin should be assessed together.
What the numbers mean
The conventional reference range for ferritin is 12–150 ng/mL. But this range is misleadingly wide. A ferritin of 12 is technically "normal" — but it's absolutely not fine. At that level, your iron stores are nearly empty.
- Depleted: below 30 ng/mL (symptoms are common)
- Suboptimal: 30–40 ng/mL
- Optimal: 40–100 ng/mL
- Elevated: above 150 ng/mL (may signal inflammation or iron overload)
Many functional and longevity-focused practitioners aim for a minimum of 40 ng/mL before they consider iron stores adequate — especially for women who are active, pregnant, or dealing with fatigue.
What affects it
- Menstruation: Monthly blood loss is the primary reason women have lower ferritin than men. Heavy periods can deplete stores rapidly.
- Diet: Iron from animal sources (heme iron) is absorbed much more efficiently than iron from plants (non-heme). Vegetarians and vegans are at higher risk of low ferritin.
- Exercise: Intense training increases iron demand. Endurance athletes, runners especially, can lose iron through sweat, foot-strike hemolysis, and GI microbleeding.
- Pregnancy and postpartum: Iron needs roughly double during pregnancy, and many women enter the postpartum period significantly depleted.
- Gut health: Conditions like celiac disease, IBD, or even low stomach acid can impair iron absorption.
- Inflammation: Ferritin is also an acute-phase protein, meaning it can be falsely elevated during infections or chronic inflammation. If your hs-CRP is high, your ferritin may look normal even when your actual iron stores are low.
How to get tested
Ferritin is a simple blood test that does not require fasting, though morning fasting samples tend to be more consistent. You can ask your doctor to add it to routine blood work — just make sure they order ferritin specifically, not just a "complete blood count" or "iron panel." It's also available through online lab services. Testing once or twice a year is usually enough, or more often if you're actively supplementing.
How to improve it
- Eat iron-rich foods. Red meat, liver, oysters, and sardines are the best sources of highly absorbable heme iron. Dark leafy greens, lentils, and fortified cereals provide non-heme iron, which is absorbed less efficiently but still helpful.
- Pair iron with vitamin C. Vitamin C significantly improves iron absorption. Take your iron supplement with orange juice or a vitamin C-rich food.
- Avoid blockers around iron intake. Coffee, tea, calcium supplements, and dairy reduce iron absorption. Keep at least a 1–2 hour gap between these and your iron-rich meals or supplements.
- Choose the right supplement form. Iron bisglycinate (also called iron glycinate) is generally easier on the stomach than ferrous sulfate. Start with 25–50 mg every other day — research shows alternate-day dosing actually improves absorption.
- Retest after 3 months. Iron stores rebuild slowly. Give supplementation at least 3 months before retesting to see meaningful improvement.