In one sentence
The protein that stores iron inside your cells, a trace of which circulates in proportion to the stores, so that a low ferritin is the earliest sign that iron is running short, long before hemoglobin falls.
Why it matters
Iron makes hemoglobin, the oxygen store in muscle and the enzymes that turn food into energy in the mitochondria, and the stores empty before the red cells suffer. In 198 women aged 18 to 53 with unexplained fatigue, a hemoglobin inside its range and a ferritin below 50 µg/L, twelve weeks of oral iron reduced the fatigue score by 48 percent against 29 percent on placebo (Vaucher 2012). A Cochrane review of 108 studies found that, at a threshold of 30 µg/L, ferritin was a reasonably sensitive (79 percent) and very specific (98 percent) test for iron deficiency in people presenting for care, on low-certainty evidence, and that a high ferritin was a weak test for iron overload (Garcia-Casal 2021). The catch is that ferritin is also an acute-phase protein: inflammation or infection raises it for weeks regardless of the iron behind it, which is why VT reads it beside CRP from the same draw and why a ferritin taken while unwell says little.
What moves it
Down: menstrual loss, pregnancy, blood donation, a diet with little meat, poor absorption in the gut, and heavy endurance training, which loses iron through sweat, the gut and the pounding of the feet. Up, regardless of iron: inflammation and infection, liver cell injury, alcohol; and with iron: supplements, a high-iron diet, and the inherited iron-loading conditions, which is the reason a high ferritin is a conversation with a doctor rather than a target to lower.
How to measure it well
No fasting needed. Not within two weeks of being unwell or of a vaccination, and not the day after a hard session; ask for CRP on the same draw so the two can be read together. High-dose biotin interfered with some ferritin immunoassays in a crossover study of healthy adults (Li 2017), so pause it for three days. Same laboratory each time.
How Vitality Tracker uses it
Stored as reported with the laboratory's range and flag, never overridden; typical laboratory reference 30 to 300 µg/L in men and 15 to 200 in women. VT's working optimal (v0.1), shown beneath in smaller type and marked working: 50 to 150 in men, 30 to 120 in women. When CRP from the same draw is raised, the ferritin is marked as read against inflammation; iron studies (iron, TIBC and transferrin saturation) are not on the panel and are added only when ferritin or CRP says so. Ferritin is listed in the fatigue and low-energy-availability clusters of the pre-visit report. Re-test when a symptom cluster shows or six months after an iron intervention, never within two weeks of being unwell.
How far to trust the number
Laboratory: an immunoassay with small analytical error. The acute-phase effect is the whole uncertainty: without a CRP beside it, a ferritin inside its range in someone fighting something can hide empty stores.
The ranges
The laboratory’s range and flag are shown with your result and never overridden; the working optimal stays beneath, in smaller type, until the clinical advisor has reviewed it.
Sources
- Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ 2012;184:1247–54. doi:10.1503/cmaj.110950
- Garcia-Casal MN, et al. Serum or plasma ferritin concentration as an index of iron deficiency and overload. Cochrane Database Syst Rev 2021;5:CD011817. doi:10.1002/14651858.CD011817.pub2
- Camaschella C. Iron-deficiency anemia. N Engl J Med 2015;372:1832–43. doi:10.1056/NEJMra1401038
- Li D, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA 2017;318:1150–60. doi:10.1001/jama.2017.13705
- Vitality Tracker, Recommended baseline lab panel v0.1; optimal set v0.1 (working).