Top Causes of Fatigue

Fatigue is one of the most common reasons people visit a doctor — and one of the most frequently dismissed with "you're just stressed" or "it's part of getting older." But persistent fatigue is a symptom, not a diagnosis, and it almost always has an identifiable cause.

Here are the most common medical causes of chronic fatigue — and what to do about them.

The thyroid gland regulates metabolism throughout the body. When thyroid hormone production is insufficient (hypothyroidism), virtually every metabolic process slows — producing fatigue, weight gain, cold intolerance, brain fog, hair loss, and depression.

Hypothyroidism is significantly more common in women and becomes increasingly prevalent with age. Subclinical hypothyroidism — where TSH is mildly elevated but T4 is still "normal" — is frequently missed on standard testing but can produce significant fatigue.

What to test: TSH, free T3, free T4, reverse T3, and thyroid antibodies (TPO and thyroglobulin). Standard TSH testing alone misses many cases.

Iron deficiency — even without frank anemia — is one of the most common causes of fatigue, particularly in women of reproductive age. Iron is essential for hemoglobin (which carries oxygen to tissues) and for mitochondrial energy production.

Symptoms include fatigue, weakness, brain fog, cold intolerance, and shortness of breath with exertion.

What to test: Ferritin (iron stores) is more sensitive than serum iron or hemoglobin for detecting early deficiency. A ferritin below 30 ng/mL is associated with fatigue even without anemia.

B12 is essential for red blood cell production, neurological function, and DNA synthesis. Deficiency produces fatigue, weakness, brain fog, and neurological symptoms (tingling, numbness).

Risk factors include: vegetarian/vegan diet, age over 50 (reduced stomach acid impairs absorption), use of metformin or proton pump inhibitors, and digestive conditions like celiac disease or Crohn's.

What to test: Serum B12 and methylmalonic acid (MMA) — MMA is a more sensitive marker of functional B12 deficiency.

Vitamin D deficiency is extremely common — affecting approximately 40% of American adults — and is a frequently overlooked cause of fatigue. Vitamin D is essential for mitochondrial function, and deficiency directly impairs cellular energy production.

What to test: 25-hydroxyvitamin D. Optimal levels are 50–80 ng/mL; many labs define "normal" as low as 20 ng/mL, which is insufficient for optimal energy.

Testosterone deficiency is a major cause of fatigue in both men and women. Testosterone supports mitochondrial energy production, muscle mass, and motivation. Low testosterone produces a deep, pervasive fatigue that doesn't improve with rest — often described as a loss of vitality.

What to test: Total testosterone, free testosterone, and SHBG. "Normal" ranges on standard labs are often too broad — symptoms matter as much as numbers.

Chronic stress dysregulates the HPA axis — the system that controls cortisol production. This can manifest as either chronically elevated cortisol (producing anxiety, insomnia, and fatigue) or blunted cortisol output (producing profound morning fatigue, difficulty handling stress, and salt cravings).

What to test: 4-point salivary cortisol testing maps the full diurnal cortisol pattern and is more informative than a single blood cortisol measurement.

Sleep apnea — in which breathing repeatedly stops during sleep — is one of the most common and most underdiagnosed causes of fatigue. It produces fragmented, non-restorative sleep regardless of how many hours you spend in bed. Many people with sleep apnea don't know they have it.

Other sleep disorders including insomnia, restless leg syndrome, and circadian rhythm disruption also produce significant fatigue.

What to test: If you snore, wake frequently, or feel unrefreshed despite adequate sleep time, a sleep study is warranted.

Insulin resistance impairs glucose delivery to cells, reducing cellular energy production. The energy crashes after meals, persistent afternoon fatigue, and difficulty concentrating that many people experience are often signs of blood sugar dysregulation.

What to test: Fasting insulin (not just glucose), hemoglobin A1c, and a comprehensive metabolic panel.

Beyond iron deficiency, other causes of anemia — including B12 deficiency, folate deficiency, and chronic disease — reduce the blood's oxygen-carrying capacity, producing fatigue, weakness, and shortness of breath.

What to test: Complete blood count (CBC) with differential.

Mental health conditions are among the most common causes of fatigue — and they're often intertwined with the physical causes listed above. Depression and anxiety produce fatigue through disrupted sleep, elevated cortisol, reduced motivation, and neurochemical changes that impair energy.

Importantly, many of the physical causes of fatigue (thyroid dysfunction, hormonal imbalances, nutritional deficiencies) also cause depression and anxiety — making it essential to evaluate both together.

The most important thing to understand about fatigue is that it almost always has a cause — and that cause is almost always identifiable with the right testing. "You're just tired" is not an acceptable answer.

At Northern Lights Wellness, our comprehensive fatigue evaluation includes thyroid, hormonal, nutritional, metabolic, and inflammatory markers — giving us a complete picture of what's driving your symptoms.

Book a Wellness Consultation to get to the bottom of your fatigue and find real solutions.