Her hemoglobin is fine. Her ferritin is 18.

A normal hemoglobin tells you she is not anemic. It does not tell you she has enough iron. Ferritin is the earlier signal, and hers is low.

Is a ferritin of 18 worth treating?

The best answer comes from a randomized trial of 198 menstruating women with unexplained fatigue, ferritin below 50 and normal hemoglobin. Women given 80 mg of elemental iron daily for 12 weeks saw fatigue scores drop by 47.7 percent, compared with 28.8 percent on placebo.

Iron deficiency does not wait for anemia to make someone tired.

The placebo effect was large, which is worth telling your patient. The difference was still real, and her ferritin rose along with it.

How you dose it matters

Daily oral iron raises hepcidin, which then blocks absorption of the next dose. In two small trials of iron-depleted women, giving iron on alternate days led to a larger share of each dose being absorbed, and more iron absorbed overall across the same number of doses. Splitting a dose into twice daily did not help.

Alternate-day dosing may also mean fewer GI side effects, which is often the real reason patients stop.

Why is she low?

Heavy menstrual bleeding and a mostly plant-based diet are both plausible here. They are not the only possibilities.

What the evidence says

The fatigue benefit comes from one well-designed trial in menstruating women, so be cautious extending it to other groups. The alternate-day absorption data are from short, small studies using isotope labels, not long-term outcomes. Both point the same way: treat low ferritin in symptomatic patients, and dose with absorption in mind.

What to do Monday

What to do

1. Treat symptomatic patients with ferritin under 50, even with a normal hemoglobin.
2. Consider alternate-day dosing for better absorption and tolerance.
3. Recheck ferritin and hemoglobin at six to eight weeks, and address the cause.