Serum Ferritin Tracks Physical Activity Capacity in a Patient with ME/CFS and POTS: A 1,894-Day Within-Person (N-of-1) Study

Background. Iron deficiency without anemia (IDNA) causes fatigue and reduced exercise tolerance, and several conditions (restless legs syndrome and heart failure) already use ferritin thresholds far above the lower limit of the standard female reference range. Whether ferritin tracks functional capacity in ME/CFS and POTS is essentially untested. Case. A 34-year-old woman (BMI 21) with a 12-year history of ME/CFS and POTS, no pregnancies or blood donation, self-recorded daily walking distance for 1,894 consecutive days (May 2021–July 2026), generally walking as much as symptoms allowed. Serum ferritin was measured 34 times over the same window (range 9–73 ng/mL). Methods. Each ferritin draw was paired with mean daily activity over the preceding 30 days, giving 34 independent observations; correlation by Pearson's r with a Fisher-z 95% confidence interval. The sparse ferritin series was deliberately not interpolated onto a daily grid, a procedure that inflates apparent significance via autocorrelation. Results. Ferritin correlated positively with trailing 30-day activity capacity: r = +0.53 (95% CI 0.24–0.74; p = 0.001; n = 34), robust across 14–90-day windows, alignments, and to the weighting of the activity metric. Capacity kept rising with ferritin well above the laboratory reference floor (~15 ng/mL): mean daily walking rose stepwise across ferritin bands, from 2.54 mi/day at <20 ng/mL to 3.32 at 20–49 and 3.67 at ≥50, a 44% gain from lowest to highest. Much of that increase fell within the range conventionally reported as normal. Abrupt ferritin changes in both directions coincided with capacity changes, functioning as within-subject natural experiments. Inflammatory markers were uniformly normal across the full ferritin range (hs-CRP <0.4 mg/L; ESR 2 mm/hr), placing the variation on the iron-storage rather than the acute-phase arm of ferritin biology. Conclusion. In this single patient, serum ferritin was a strong and consistent longitudinal predictor of physical activity capacity, with benefit extending above the conventional lower reference limit. The findings motivate a prospective cohort study analyzing ferritin and objectively-measured activity within individuals.