Why Am I Tired All the Time? 10 Medical Causes of Constant Fatigue
From iron deficiency to sleep apnea — the treatable reasons you're always exhausted
Constant fatigue is one of the most common reasons people visit a doctor, affecting an estimated 20% of adults. In most cases, the cause is identifiable and treatable through targeted lab testing — not just 'get more sleep.'
"Why am I so tired?" is the #1 health-related Google search in the US. If fatigue is your baseline state despite getting adequate sleep, something physiological is draining your energy — and standard bloodwork often misses it.
Most doctors order a CBC and basic metabolic panel, which catches anemia and kidney/liver issues but misses the 10 most common treatable causes of fatigue: subclinical thyroid dysfunction, low ferritin (without anemia), insulin resistance, cortisol dysregulation, vitamin D deficiency, sleep-disordered breathing, hormonal imbalance, magnesium deficiency, chronic inflammation, and B12 deficiency.
10 Treatable Causes of Constant Fatigue
- Low ferritin (without anemia) — Ferritin below 50 ng/mL impairs energy even when hemoglobin is normal. Affects 30% of premenopausal women. Standard labs only flag ferritin below 12.
- Subclinical hypothyroidism — TSH between 2.5–4.5 mIU/L with low-normal free T3 causes fatigue, weight gain, and brain fog — but is often dismissed as 'normal.'
- Insulin resistance — Impaired cellular glucose uptake starves cells of energy despite adequate food intake. Fasting insulin (not glucose) catches it early.
- Vitamin D deficiency — Levels below 30 ng/mL affect an estimated 42% of US adults. Optimal for energy: 50–80 ng/mL.
- Sleep apnea — 80% of moderate-to-severe cases are undiagnosed. Fragments sleep architecture and reduces restorative sleep stages.
- Cortisol dysregulation — Flat cortisol curve from chronic stress impairs the natural morning energy surge.
- Low testosterone — Affects 20–40% of men over 45 and women in perimenopause. Directly impacts energy, mood, and motivation.
- Magnesium deficiency — Required for 300+ enzymatic reactions including ATP production. An estimated 50% of Americans are deficient.
- Chronic inflammation — Elevated hs-CRP (>1.0 mg/L) from diet, obesity, infection, or autoimmunity directly causes fatigue.
- B12 deficiency — Common in vegetarians, those over 50 (reduced absorption), and metformin users. Optimal >500 pg/mL, not just >200.
The Fatigue Workup: Tests to Request
| Test | What It Reveals | Optimal Range |
|---|---|---|
| Ferritin | Iron stores | 50–150 ng/mL |
| TSH + Free T3 + Free T4 | Thyroid function | TSH 0.5–2.0 mIU/L |
| Fasting insulin | Metabolic function | <7 μIU/mL |
| Vitamin D (25-OH) | Vitamin D status | 50–80 ng/mL |
| AM cortisol | Adrenal function | 10–18 μg/dL (8 AM) |
| Total + Free testosterone | Hormonal status | Age/sex appropriate |
| RBC Magnesium | Intracellular Mg | 5.0–6.5 mg/dL |
| hs-CRP | Inflammation | <1.0 mg/L |
| B12 | Neurological health | >500 pg/mL |
| CBC | Anemia, infection | Standard ranges |
Frequently Asked Questions
Why am I always tired no matter how much I sleep?
The most common causes are: undiagnosed sleep apnea (80% of cases are undiagnosed), low ferritin without anemia (ferritin below 50 ng/mL), subclinical thyroid dysfunction (TSH above 2.5 with low free T3), and insulin resistance. Standard bloodwork typically misses all four. Request a comprehensive panel including ferritin, thyroid panel with free T3, fasting insulin, and consider a sleep study.
What deficiency causes extreme fatigue?
The most common deficiencies causing fatigue are: iron/ferritin (especially in women), vitamin D, vitamin B12, and magnesium. Low ferritin is the single most common nutrient deficiency linked to fatigue — and standard labs often report it as 'normal' at levels that are functionally insufficient for energy production.
When should I see a doctor about fatigue?
See a doctor if fatigue persists for more than 4 weeks despite adequate sleep and basic lifestyle optimization, or immediately if accompanied by unexplained weight loss, persistent fever, severe headaches, chest pain, or new neurological symptoms. Fatigue lasting more than 6 months warrants a comprehensive workup.
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