Chronic Fatigue: Possible Causes and Medical Evaluation

Feeling exhausted despite getting enough sleep is one of the most common, and most frustrating, health complaints. But fatigue is a symptom, not a diagnosis. It can be caused by dozens of different medical conditions, and pinpointing the right one makes the difference between a treatment that works and years of unresolved tiredness.

Why the Root Cause Matters

Fatigue can stem from hormonal imbalances, thyroid dysfunction, sleep disorders, vitamin and mineral deficiencies, anemia, depression and other mood disorders, metabolic conditions like insulin resistance, chronic infections, and in some cases, underlying malignancy. Some patients have more than one contributing factor, which is why a surface-level workup often misses the true cause. A quick lab panel or a five-minute visit is rarely enough. The right evaluation starts with a detailed history, a thorough review of your symptoms, and targeted testing based on what your physician suspects. Identifying a treatable contributor can help guide care, but fatigue may have several overlapping causes, and testing may not identify a single explanation. Below are conditions a thorough fatigue workup may consider.

Vitamin and Mineral Deficiencies

Deficiencies in key nutrients are among the most common and most treatable causes of fatigue. Iron deficiency is the most prevalent nutritional deficiency worldwide. Even without full-blown anemia, low iron stores (measured by ferritin) can cause fatigue, brain fog, and exercise intolerance. Vitamin B12 deficiency is common in older adults, vegetarians, and patients on certain medications like metformin. It can cause fatigue, weakness, numbness, and cognitive changes. Vitamin D deficiency affects an estimated 42% of U.S. adults and is associated with fatigue, muscle weakness, and mood changes. Low magnesium and folate can also contribute to persistent tiredness. The good news is that once identified, these deficiencies are often straightforward to correct with targeted supplementation and dietary changes.

Anemia

Anemia (low red blood cells or low hemoglobin) means your blood carries less oxygen to your tissues, resulting in fatigue, weakness, dizziness, shortness of breath, and pallor (unusually pale skin, nail beds, or inner eyelids). Anemia has many causes of its own, including iron deficiency, B12 or folate deficiency, and chronic disease. Bleeding is another important cause that must be evaluated. This includes obvious sources like heavy menstrual periods, but also less apparent causes such as gastrointestinal bleeding from ulcers, polyps, or other conditions that may not produce visible symptoms. In rarer cases, bone marrow disorders can be responsible. A complete blood count (CBC) with iron studies, ferritin, B12, and folate levels helps identify both the presence and the underlying cause of anemia so treatment can be properly targeted.

Thyroid Dysfunction

Your thyroid gland produces hormones that control your metabolism, essentially how fast or slow your body runs. An underactive thyroid (hypothyroidism) is one of the most well-known causes of fatigue, along with weight gain, cold intolerance, constipation, dry skin, and hair thinning. However, subtle thyroid dysfunction can be missed if only a basic TSH test is run. A comprehensive thyroid panel including TSH, free T4, free T3, and thyroid antibodies gives a fuller picture and can identify autoimmune thyroid disease (Hashimoto's thyroiditis) that basic testing would miss.

Depression and Mood Disorders

Fatigue is one of the most common symptoms of depression, and the two conditions frequently coexist. Depression-related fatigue can feel physical: heavy limbs, difficulty getting out of bed, and a persistent lack of motivation that goes beyond feeling sad. Anxiety disorders can also drive fatigue through constant mental and physical tension, poor sleep, and an overactive stress response. It is important to screen for mood disorders as part of any chronic fatigue evaluation, because treating depression or anxiety often leads to significant improvement in energy levels. At Thrive RX, we evaluate the whole person, including mental health, as part of our root-cause approach.

Hormonal Imbalances

Declining or imbalanced hormones are a significant cause of fatigue across genders. In women, dropping estrogen and progesterone levels during perimenopause and menopause commonly cause fatigue, sleep disruption, brain fog, and mood changes. In men, declining testosterone (which begins gradually after age 30) can cause fatigue, reduced motivation, loss of muscle mass, and low mood. Cortisol imbalance from chronic stress is another hormonal driver of fatigue. When cortisol (your body's primary stress hormone) stays elevated from ongoing stress or becomes depleted, your energy regulation breaks down, leading to afternoon crashes, morning grogginess, and a feeling of being 'wired but tired.'

Sleep Disorders and Sleep Hygiene

Many people assume that because they spend 7-8 hours in bed, they are getting adequate rest. But the quality of your sleep matters as much as the quantity. Obstructive sleep apnea (OSA) is one of the most common and most underdiagnosed causes of chronic fatigue, with an estimated 80% of moderate-to-severe cases going undiagnosed. When your airway partially or fully collapses during sleep, your body is repeatedly pulled out of deep, restorative sleep stages, even if you don't wake up fully. Untreated sleep apnea also raises cortisol, promotes weight gain, elevates blood pressure, and increases cardiovascular risk. Other sleep disorders including insomnia, restless leg syndrome, and disrupted circadian rhythm can also contribute. Beyond diagnosable sleep disorders, poor sleep hygiene is a surprisingly common and correctable contributor to fatigue. Sleep hygiene refers to the habits and environmental factors that promote consistent, high-quality rest. Inconsistent sleep schedules, excessive screen time before bed, caffeine later in the day, alcohol close to bedtime, and a warm or noisy bedroom can all degrade sleep quality without the person realizing why they feel tired. Simple changes, such as keeping a consistent sleep and wake time, limiting blue light exposure in the evening, keeping the bedroom cool and dark, and establishing a wind-down routine, can meaningfully improve how rested you feel.

Metabolic Conditions

Insulin resistance (where your cells stop responding properly to insulin, so sugar can't get into cells efficiently for fuel) is an increasingly common cause of fatigue, especially in patients who also struggle with weight resistance. Prediabetes and type 2 diabetes both cause fatigue through dysregulated blood sugar, producing energy spikes and crashes throughout the day. A comprehensive metabolic panel, including fasting glucose and hemoglobin A1c, along with a thorough clinical assessment, can identify these conditions even in early stages when they are most responsive to treatment.

Underlying Malignancy

While less common than the other causes listed here, unexplained fatigue that is persistent, progressive, or accompanied by unintentional weight loss, night sweats, or other new symptoms can sometimes be an early sign of cancer. Certain cancers, including lymphoma, leukemia, and some solid tumors, can present with fatigue as one of the earliest symptoms. This is not meant to cause alarm, but it is one of the reasons a comprehensive physician evaluation matters: to make sure serious conditions are not overlooked. Age-appropriate cancer screenings and targeted lab work help rule out or identify malignancy early when outcomes are best.

Other Contributing Factors

The causes above account for the majority of chronic fatigue cases, but several other conditions belong on the list:

• Autoimmune diseases such as lupus, rheumatoid arthritis, and multiple sclerosis
• Chronic infections including Epstein-Barr virus and Lyme disease
• Post-viral fatigue and long COVID
• Medication side effects (blood pressure medications, antihistamines, antidepressants, and others)
• Chronic inflammatory conditions

What a Thorough Evaluation Looks Like

At many practices, a fatigue workup begins and ends with a CBC and TSH. Those are important starting points, but they only scratch the surface. At Thrive RX, Dr. Kiswani-Barley starts with a detailed medical history and symptom review, then selects thyroid, hormone, nutrient, metabolic, and inflammatory tests when appropriate, along with sleep evaluation and mental health screening. The goal is to investigate possible contributors, not just the first abnormality that appears. Testing is one part of the evaluation and cannot identify every contributing factor. When results point to a condition that requires specialty care, such as a hematologic disorder, autoimmune disease, or suspected malignancy, she coordinates referrals to trusted specialists and continues to manage your overall care.

What Research Shows

Studies show that up to 40% of patients with chronic fatigue have an undiagnosed underlying cause that improves significantly once identified and properly treated. The range of possibilities, from thyroid dysfunction and anemia to depression and malignancy, is exactly why a thorough workup matters. Sleep apnea alone affects an estimated 30 million Americans. Iron deficiency affects up to 10% of women of reproductive age. Depression is the leading cause of disability worldwide. These are common, treatable conditions, and they are frequently missed when evaluations are rushed or incomplete.