Why Am I So Tired All The Time?

Feeling tired all the time is one of the most common complaints in general practice. Everyone has weeks where they're running on empty, and that's usually explained by poor sleep, overwork, or stress. The kind of tiredness worth investigating is the type that doesn't improve with rest, lasts more than a few weeks, and starts affecting your ability to concentrate, work, or enjoy normal activities.

In most cases, the cause is identifiable and treatable. A blood test can pick up the majority of medical causes within 24-48 hours.

Common medical causes

Iron deficiency

The single most common medical cause of fatigue, especially in women of reproductive age. Heavy periods, pregnancy, and a diet low in red meat or iron-rich foods all deplete iron stores. You can be iron-deficient without being technically anaemic — your haemoglobin may be normal while your ferritin (stored iron) is low.

Symptoms include tiredness, brain fog, breathlessness on exertion, pale skin, and brittle nails. Treatment with iron supplements usually produces noticeable improvement within 2-4 weeks.

Thyroid dysfunction

An underactive thyroid (hypothyroidism) slows your metabolism, causing tiredness, weight gain, cold sensitivity, constipation, and dry skin. It affects about 2% of the UK population and is more common in women over 40.

A simple blood test measuring TSH (thyroid-stimulating hormone) and free T4 confirms the diagnosis. Treatment with levothyroxine is straightforward and lifelong, but effective.

Vitamin D deficiency

Widespread in the UK, particularly between October and March when sunlight is insufficient for skin to produce vitamin D. Deficiency causes fatigue, muscle aches, and low mood. Public Health England recommends all adults consider supplementing with 10 micrograms (400 IU) daily during autumn and winter.

A blood test confirms your level. If it's significantly low, your GP may prescribe a loading dose before moving to maintenance supplementation.

Vitamin B12 deficiency

More common in vegetarians, vegans, and older adults. Symptoms overlap with iron deficiency: fatigue, weakness, tingling in the hands or feet, and difficulty concentrating. Pernicious anaemia (an autoimmune condition affecting B12 absorption) is the most serious form and requires B12 injections rather than oral supplements.

Diabetes

Both type 1 and type 2 diabetes cause fatigue, often as one of the earliest symptoms. When your body can't use glucose properly, your cells are literally starved of fuel. Increased thirst, frequent urination, and unexplained weight changes alongside tiredness should prompt a blood sugar check (HbA1c test).

Depression and anxiety

Mental health conditions cause physical fatigue. Depression in particular causes a bone-deep tiredness that sleep doesn't fix. If your fatigue is accompanied by persistent low mood, loss of interest in things you normally enjoy, changes in appetite, or difficulty sleeping, depression may be a factor.

This doesn't mean the tiredness is "all in your head" — the fatigue of depression has a neurochemical basis and is entirely real.

Lifestyle causes worth ruling out

Before assuming a medical cause, consider the basics. Poor sleep quality (from screen use, irregular schedules, or sleep apnoea) is a major contributor. Alcohol disrupts restorative sleep even in moderate amounts. Caffeine after midday affects sleep architecture more than most people realise.

Lack of physical activity, counterintuitively, causes fatigue rather than preventing it. Regular moderate exercise consistently improves energy levels in clinical trials. If you're sedentary and tired, starting with 20-30 minutes of walking daily is one of the most evidence-backed interventions available.

How we can help

Our fatigue assessment is designed to systematically investigate persistent tiredness. It includes a comprehensive blood panel covering iron studies, thyroid function, vitamin D, B12, folate, HbA1c, liver and kidney function, and inflammatory markers.

Results are typically available within 24-48 hours. Your GP will review them with you, explain what's been found, and start treatment if needed — whether that's a prescription, supplementation plan, or onward referral. For a broader investigation that combines fatigue markers with cardiovascular, metabolic, and organ screening, consider our Full Body MOT health check.