What causes persistent fatigue in adults?
Persistent adult fatigue is rarely a single cause. The dominant vectors are hormonal (low testosterone, thyroid drift, cortisol dysregulation, perimenopause), metabolic (insulin resistance, anemia, B12/iron/vitamin D deficiency), sleep architecture (apnea, fragmentation), and lifestyle load. A biomarker panel separates the vectors before any pathway is recommended.
Full answer
Persistent fatigue almost never traces back to a single cause. The clinically dominant vectors are hormonal (low testosterone, hypothyroidism, subclinical thyroid drift, perimenopause and menopause, cortisol dysregulation), metabolic (insulin resistance, anemia, iron deficiency with low ferritin, B12 deficiency, vitamin D deficiency, fatty liver), sleep (obstructive sleep apnea, poor sleep architecture, circadian misalignment), and lifestyle load (chronic underfeeding, overtraining, alcohol, medications). The standardized biomarker panel — hormones, CBC (with ferritin and iron studies where indicated), CMP, lipids, plus thyroid (TSH, fT3, fT4, reverse T3), B12, vitamin D, and hs-CRP — is read together with symptoms, sleep history, and training context to separate the vectors. Treating fatigue without separating the vector is how patients end up on the wrong pathway.
Educational. Not a substitute for individualized provider review.
- Symptoms & side effects — Primary source · oMeds
- Fatigue & recovery — Reference · oMeds Learn
- Thyroid intelligence — Reference · oMeds Learn
- Symptoms reference — Reference · oMeds Learn
- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29