Ferritin is your body’s iron storage protein. It acts like a vault, locking iron away in a safe form inside your cells and releasing it when your body needs more. The amount of ferritin circulating in your blood reflects how much iron you have in reserve, which is why a ferritin blood test is the most common way to check your iron status.
How Ferritin Stores Iron
Iron is essential for carrying oxygen in your blood, powering your muscles, and supporting brain function. But free iron floating around in your cells is actually toxic. It triggers a chemical reaction that damages cell membranes and DNA through oxidative stress. Ferritin solves this problem by trapping iron in a safe, non-reactive form.
Each ferritin molecule is built from 24 protein subunits that snap together into a hollow, nearly spherical shell. The cavity inside is about 8 nanometers across, tiny but capable of holding up to 4,000 iron atoms stored as a mineral called ferrihydrite. Iron enters and exits through small pores along the shell’s surface, specifically through water-friendly channels that allow controlled movement in and out.
Ferritin exists in virtually every cell in your body, but the largest concentrations sit in your liver, spleen, and bone marrow. A small amount of ferritin leaks into your bloodstream, and this “serum ferritin” is what gets measured in a blood test. It tracks closely with your total stored iron under normal conditions: low serum ferritin means your reserves are running thin, and high serum ferritin means your body is holding onto a lot of iron (or, as explained below, responding to inflammation).
How Your Body Controls Iron Release
Your body doesn’t just passively store and dump iron. It actively regulates the flow using a hormone called hepcidin, produced mainly by your liver. Hepcidin controls a doorway protein called ferroportin, which sits on the surface of cells that store or absorb iron. Ferroportin is the only known exit route for iron to leave a cell and enter your bloodstream.
When your iron levels are high, your liver ramps up hepcidin production. Hepcidin latches onto ferroportin and causes the cell to pull it inside and destroy it. With fewer exit doors, iron stays locked in storage and less gets absorbed from food. When your iron stores drop, hepcidin production falls. Ferroportin stays active on cell surfaces, iron flows freely out of storage and into circulation, and your gut absorbs more iron from meals. This feedback loop keeps your blood iron levels remarkably stable day to day, even if your diet fluctuates.
What Ferritin Levels Tell You
A ferritin blood test is the single best snapshot of your iron reserves. Normal ranges vary by sex and age:
- Adult females: 15 to 205 ng/mL
- Adult males: 30 to 566 ng/mL
- Children (6 months to 15 years): 12 to 140 ng/mL
- Newborns: levels can be as high as 650 ng/mL in the first six months, then gradually fall
The World Health Organization defines iron deficiency as a ferritin level below 15 micrograms per liter in adults and children over age 5. For infants and preschool children, the cutoff is below 12. These thresholds shift upward when someone has an infection or inflammation: below 70 for adults and school-age children, below 30 for infants and preschoolers. The reason for that adjustment is explained in the next section.
Some researchers argue these traditional cutoffs are too low and miss a large number of people who are genuinely iron deficient. A 2025 analysis found that diagnostic accuracy improved significantly when the cutoff was raised to 45 for females and 70 for males, catching 44% more iron-deficient women who would have been missed at the older threshold.
Why Ferritin Isn’t Always About Iron
Here’s where ferritin gets tricky. It’s also an acute-phase reactant, meaning your body produces more of it during inflammation, infection, or immune activation, regardless of how much iron you actually have stored. When your immune system fires up, inflammatory signals push your cells (especially immune cells called macrophages) to churn out extra ferritin. This can mask iron deficiency: your ferritin looks normal or even elevated on a blood test, but your actual iron reserves are low.
This is a common diagnostic pitfall in people with chronic conditions like autoimmune disease, kidney disease, heart failure, or active infections. In these situations, a ferritin level that looks “fine” may be misleading. That’s why doctors often order additional tests alongside ferritin, particularly transferrin saturation (TSAT), which measures how much of the iron-carrying protein in your blood is actually loaded with iron. TSAT below 20% suggests your tissues aren’t getting enough iron, even if your ferritin number looks reassuring. Research in heart failure patients has found that low TSAT predicts poor outcomes more reliably than ferritin alone.
Low Ferritin Without Anemia
Iron deficiency progresses in stages. First, your ferritin drops as stored iron depletes. Then, the iron circulating in your blood decreases. Only in the final stage does your hemoglobin fall, and that’s when you’re officially anemic. Many people get stuck in that first or second stage for months or years, with a normal blood count but genuinely depleted reserves.
These people often feel terrible. Fatigue, brain fog, difficulty concentrating, muscle and joint pain, headaches, shortness of breath, palpitations, and restless legs are all documented symptoms of iron deficiency even when hemoglobin is completely normal. Because standard blood counts come back fine, these patients frequently get misdiagnosed with chronic fatigue syndrome, fibromyalgia, burnout, or thyroid problems. One clinical review noted that menstruating women in particular may cycle through these symptoms for years before anyone checks a ferritin level.
For restless leg syndrome specifically, a ferritin below 75 is considered the threshold for iron deficiency contributing to symptoms, well above the traditional “normal” cutoff of 15. In heart failure, iron deficiency impairs quality of life when ferritin is below 100, again far above the level most labs flag as low.
What Very High Ferritin Means
On the other end of the spectrum, ferritin above 300 ng/mL in men or above 150 to 200 ng/mL in women warrants investigation. The cause is sometimes genuine iron overload. Hereditary hemochromatosis, a genetic condition where the body absorbs too much iron from food, can push ferritin above 1,000 ng/mL if untreated. Over time, the excess iron deposits in organs like the liver, heart, and pancreas and causes damage.
But more often, very high ferritin reflects inflammation rather than excess iron. Ferritin above 500 ng/mL is a hallmark of several serious inflammatory conditions, including severe infections, septic shock, and certain autoimmune flare-ups. Levels above 3,000 raise strong suspicion for a dangerous immune overreaction where the body’s own immune cells begin attacking its tissues. Context matters enormously: a ferritin of 800 in someone feeling well points toward a different workup than a ferritin of 800 in someone with a high fever and crashing blood pressure.
How Ferritin Fits Into a Full Iron Panel
Ferritin is typically ordered as part of a broader set of iron-related blood tests. Each test captures a different piece of the puzzle:
- Serum ferritin reflects total iron stores in your body.
- Serum iron measures the iron currently circulating in your blood. This number fluctuates throughout the day and spikes after meals or iron supplements, making it less reliable on its own.
- Transferrin saturation (TSAT) shows what percentage of your blood’s iron-carrying capacity is being used. It’s a better indicator of whether iron is actually available to your cells right now.
- Hemoglobin and MCV (mean corpuscular volume, the size of your red blood cells) reveal whether iron deficiency has progressed far enough to affect red blood cell production. Small, pale red blood cells are the classic late sign of iron deficiency anemia.
Ferritin is the most useful single test for catching iron problems early, before anemia develops. But because inflammation can distort it in either direction, the full picture usually requires at least ferritin and TSAT together, especially in anyone with a chronic health condition.