Ferritin: iron stores, with an inflammation problem

Ferritin is the protein cells use to store iron. A small amount leaks into the blood in proportion to the body's stores, so serum ferritin is the standard way to estimate them. It is reported in ng/mL or µg/L, which are the same number. Ferritin is also an acute-phase protein: it rises with inflammation, infection, liver injury and some cancers whatever the iron status. That is why Peptide AI groups it with the inflammatory markers, and why it is never read alone.

The guideline numbers

The WHO says ferritin alone should not be used to identify iron overload, and a high value should prompt a work-up for the cause. Lab reference ranges vary widely, often running from the low teens to 300 or more in men. The ferritin "sweet spots" promoted online for energy or hair are opinion; the guideline cutoffs above are about deficiency and overload.

What raises and lowers it

• Lowers: blood loss (heavy periods, blood donation, gut bleeding), low iron intake, poor absorption (coeliac disease, some gut surgery, acid-suppressing drugs), pregnancy, and endurance training.
• Raises with real iron excess: hereditary haemochromatosis, repeated transfusions, excess supplements.
• Raises without iron excess: inflammation and infection, fatty liver disease and other liver injury, heavy alcohol use, obesity and metabolic syndrome.

Because inflammation pushes ferritin up, a "normal" ferritin in someone with high hs-CRP can hide iron deficiency. Transferrin saturation, which falls in deficiency and rises in overload, is the usual companion test.

What it predicts

Low ferritin predicts iron-deficiency anaemia, and iron deficiency can cause fatigue and reduced exercise capacity before haemoglobin falls. At the other end, a 2013 meta-analysis of 12 prospective studies with 185,462 people found that those in the top fifth of ferritin had a 1.73 times higher risk of developing type 2 diabetes than those in the bottom fifth. That is an observational association, and some of it is likely fatty liver and inflammation showing up in ferritin rather than iron causing diabetes.

On GLP-1 drugs and weight loss

The pivotal semaglutide and tirzepatide trials do not report ferritin as an outcome. Two indirect effects are worth knowing. Eating much less means taking in less iron, which matters most for people who menstruate or already had low stores; the micronutrients during weight loss page covers this. And because weight loss lowers inflammation and liver fat, a ferritin that was high for those reasons can drift down. Either direction can be real, which is the argument for a baseline before starting.

How to test

• Morning draw, ideally fasting, usually with a full blood count and transferrin saturation. Iron levels themselves swing through the day and after meals; ferritin is steadier.
• Be well. A recent infection can raise ferritin for weeks. Check hs-CRP on the same draw.
• Stop iron supplements for a few days beforehand if your clinician wants to see untreated levels.
• Endurance athletes: a long, hard session the day before can affect iron markers.

Tracking it in Peptide AI

In Peptide AI, ferritin sits in the inflammatory panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. If you track iron supplements in the supplement scanner, they sit on the same timeline. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.