Why Am I Always Tired? 10 Common Causes of Fatigue

Fatigue is one of the most common symptoms encountered in clinical practice, but also one of the most misunderstood.

Feeling persistently tired doesn't necessarily mean you're not getting enough sleep. Fatigue can result from a wide range of physiological, psychological and lifestyle factors, often with several contributing simultaneously.

The important thing is to understand why you're tired, rather than simply looking for something to give you more energy.

1. Poor Sleep Quality

Sleep duration is only part of the picture. Conditions such as obstructive sleep apnoea, insomnia and restless legs syndrome can significantly impair sleep quality, leaving you exhausted despite spending eight hours or more in bed.

2. Chronic Stress and Mental Health

Prolonged psychological stress, anxiety, depression and burnout can all contribute to persistent fatigue. The relationship is often bidirectional, with poor sleep and physical exhaustion further affecting psychological wellbeing.

3. Iron Deficiency and Anaemia

Iron is essential for haemoglobin production and oxygen transport. Iron deficiency, with or without anaemia, can contribute to fatigue, reduced exercise tolerance and impaired concentration.

4. Thyroid Dysfunction

Thyroid hormones regulate numerous metabolic processes. Hypothyroidism, where the thyroid produces insufficient hormone, commonly causes fatigue alongside symptoms such as cold intolerance, constipation and weight gain.

5. Testosterone Deficiency

Testosterone deficiency can contribute to fatigue, reduced motivation, impaired concentration and loss of muscle mass. However, fatigue alone is not sufficient to diagnose hypogonadism. Diagnosis requires compatible symptoms alongside consistently low testosterone concentrations, interpreted within the wider clinical picture.

6. Nutritional Deficiencies

Deficiencies in vitamin B12, folate and, in some circumstances, vitamin D may contribute to fatigue or associated symptoms. Dietary intake, absorption and underlying medical conditions all warrant consideration.

7. Blood Glucose Dysregulation

Diabetes and poorly controlled blood glucose can cause fatigue through several mechanisms, including altered energy metabolism, dehydration and disrupted sleep. Insulin resistance may also coexist with other contributors to fatigue.

8. Chronic Inflammation and Underlying Disease

Persistent inflammatory conditions, autoimmune disorders, chronic infections and other systemic illnesses can all produce significant fatigue, sometimes before more specific symptoms become apparent.

9. Medication and Substance Effects

Certain antihistamines, antidepressants, beta-blockers and sedative medications can contribute to tiredness. Alcohol and recreational drugs can also interfere with sleep architecture and daytime alertness.

10. Cardiovascular, Kidney and Liver Disease

Conditions such as heart failure, chronic kidney disease and significant liver dysfunction can affect energy levels through impaired physiological function, metabolic disturbances and reduced exercise tolerance.

Not every cause of fatigue is a disease.

Excessive training without adequate recovery, prolonged calorie restriction and insufficient nutritional intake can all compromise physical performance and recovery.

Hormonal changes during perimenopause and menopause may also contribute, particularly through disrupted sleep and vasomotor symptoms.

In men, low oestradiol can affect aspects of sexual function, mood and musculoskeletal health, although its independent contribution to fatigue remains uncertain.

Another important consideration is myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), particularly where fatigue is accompanied by post-exertional malaise, unrefreshing sleep and cognitive difficulties.

Blood investigations can be helpful when fatigue is persistent or unexplained, but they should be guided by the clinical history, examination and associated symptoms.

Depending on the presentation, initial investigations may include:

• Full blood count and ferritin: To investigate anaemia and iron deficiency.
• Thyroid function: TSH, with free T4 where indicated.
• Kidney and liver function: To assess renal function, electrolytes and hepatic markers.
• HbA1c: To assess longer-term blood glucose regulation.
• Calcium: To identify relevant metabolic abnormalities.
• Vitamin B12 and folate: Where deficiency is suspected.
• Vitamin D: Where clinically indicated, rather than as a routine investigation for everyone.
• Testosterone, SHBG and calculated free testosterone: In men with symptoms or clinical features suggestive of testosterone deficiency.
• Inflammatory markers: CRP and, where appropriate, ESR.

Importantly, normal blood results do not exclude sleep disorders, psychological causes, medication effects or conditions such as ME/CFS.

Fatigue is a symptom, not a diagnosis.

There is an important distinction between feeling sleepy, experiencing physical exhaustion, lacking motivation and becoming unusually breathless or fatigued during exercise. These symptoms may sound similar, but they can point towards very different underlying problems.

A comprehensive assessment considers sleep, lifestyle, medical history, medication, psychological wellbeing and relevant biochemical markers.

The objective isn't simply to identify an abnormal blood result. It's to understand whether that finding is clinically significant and whether it actually explains the symptoms.

Treat the underlying cause, not just the symptom.