Why Am I Tired All the Time? 10 Medical Causes of Constant Fatigue
Medically reviewed by Medical Advisory Board Last reviewed 2026-05-13
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.
Topic updates
Get the weekly recovery roundup
Sleep, HRV, fatigue, burnout, cortisol rhythm, sleep apnea, overtraining, and stress physiology.
Check Where You Stand
Take our free health assessment to understand your metabolic, hormonal, and recovery risk factors — and get personalized recommendations.
Take the Free Assessment →Free · Takes 5 minutes · Instant results
Continue Reading
-
Sleep apnea, fatigue, weight gain, and metabolic risk
Connect sleep apnea to fatigue, weight gain, testing, and treatment options.
-
Chronic fatigue: causes beyond just sleep
Causes of persistent fatigue beyond sleep — iron, thyroid, inflammation.
-
Sleep apnea symptoms: signs you're not breathing at night
OSA warning signs: snoring, witnessed apneas, daytime sleepiness.
-
Waking up tired every day? Here's why
Why 7-8 hours isn't enough when sleep quality is poor.
-
Sleep inertia: why you feel groggy when you wake up
Morning grogginess — circadian, cortisol, and sleep-stage causes.
-
Afternoon energy crash: causes and how to prevent it
The 2-4 PM slump: blood sugar, cortisol, and circadian factors.
-
Chronic fatigue syndrome (ME/CFS): what we know
ME/CFS: diagnostic criteria, current research, and management.
-
Poor sleep quality: root causes and evidence-based fixes
Root causes and evidence-based fixes beyond sleep hygiene.
-
Low HRV: what it means and how to improve it
What low HRV means and how to improve autonomic recovery.
-
Cortisol and sleep: how stress hormones wreck your rest
The cortisol-melatonin axis and 'wired but tired' patterns.