Most chronic fatigue has a biological cause that shows up on basic labs. Before assuming stress or sleep is the problem, it's worth ruling out iron deficiency, vitamin D and B12 status, thyroid function and methylation issues — particularly if the fatigue persists despite normal sleep.
Strong evidence · Last reviewed:
Key facts
Top biomarker
Ferritin < 50 ng/mL
Other key labs
25(OH)D, B12, TSH, free T3
Common genetic links
MTHFR, HFE, COMT
Most common biomarker drivers
Iron deficiency — ferritin below 50 ng/mL often causes symptoms before anemia develops
Vitamin D below 30 ng/mL — fatigue and muscle weakness are early signs
B12 deficiency — pair B12 with MMA for true status
Subclinical hypothyroidism — TSH > 2.5 with low-normal free T3
High homocysteine — flags methylation inefficiency, often MTHFR-related
Genetic context
MTHFR C677T/A1298C — impaired methylation, often with high homocysteine
HFE C282Y/H63D — iron overload in some, but heterozygotes occasionally run lower iron
COMT Met/Met — slower catecholamine clearance, fatigue under stress is common
VDR variants — same blood 25(OH)D can feel insufficient to tissues
Practical next steps
Run a complete blood count, ferritin, 25(OH)D, B12, TSH, free T3 and homocysteine
Address sleep quality (apnea screen if snoring or AM headache)
Investigate gut absorption if multiple nutrients run low
Discuss results with a clinician — fatigue with weight change, night sweats or red-flag symptoms deserves prompt evaluation
Related in the knowledge graph
Biomarkers
Ferritin — Iron deficiency (ferritin < 50) is one of the most common reversible causes of chronic fatigue. Strong evidence
25(OH)D — Low vitamin D commonly presents with fatigue and muscle weakness. Moderate evidence
FAQ
Can low ferritin cause fatigue without anemia?
Yes. Many people feel symptomatic when ferritin drops below 50 ng/mL even with normal hemoglobin. Repleting iron stores typically resolves it.
Does MTHFR cause fatigue?
Indirectly — by impairing methylation when B-vitamin intake is suboptimal. Correcting folate and B12 status often improves energy in MTHFR carriers with high homocysteine.
References
Camaschella, NEJM 2015 — PMID 25946282. Iron deficiency without anemia is a common cause of fatigue and is corrected by iron repletion.