Clinical Workup Biomarkers: Exclusion and Comorbidity

A critical category of biomarkers in ME/CFS evaluation are those used not to diagnose ME/CFS but to: (a) exclude common mimics, (b) identify actionable comorbidities, and (c) complete the diagnostic picture. Normal results from these tests are informative and expected in ME/CFS; abnormal results identify conditions requiring independent management.

1 Thyroid Function Panel

Thyroid function assessment (TSH, Free T3, Free T4) is mandatory in any ME/CFS workup to exclude hypothyroidism, the most important and common mimic of ME/CFS fatigue. Hashimoto’s thyroiditis (identified by elevated thyroid peroxidase antibodies, TPO-Ab) represents both a mimic and a comorbidity: it can coexist with ME/CFS without causing the full symptom profile, and its identification enables targeted management. Thyroid function is typically normal in ME/CFS; however, normal thyroid function is a necessary component of diagnostic certainty rather than a dismissible negative finding.

2 Ferritin (Iron Stores)

Iron deficiency, defined by low serum ferritin even with normal hemoglobin and serum iron, is a common, actionable, and frequently missed contributor to fatigue. Standard laboratory reference ranges for ferritin (typically \(\geq 12\)–15 ng/mL) are insufficiently sensitive for symptom-resolution purposes: most ME/CFS-experienced clinicians use a functional target of \(\geq 50\) ng/mL for ferritin, consistent with evidence that symptoms persist in iron-deficient states below this level. Ferritin should be included in all ME/CFS workups; iron repletion to this target is low-risk and may provide meaningful symptom benefit in deficient patients.

3 Vitamin D (25-OH Vitamin D)

Vitamin D deficiency is highly prevalent in ME/CFS, exacerbated by activity limitations, indoor confinement, and sun avoidance. 25-OH vitamin D has immune-modulating effects mechanistically relevant to ME/CFS pathophysiology, including regulation of T-cell function and cytokine production. Standard thresholds apply: deficiency \(<\) 20 ng/mL, insufficiency 20–29 ng/mL. Correction of deficiency is recommended as standard of care.