Always Tired? Experts Reveal 12 Common Causes of Fatigue and When to Get Your Thyroid Checked

Persistent exhaustion is among the most frequent problems patients bring to their GP.

Around one in five people in Britain report feeling tired all the time — yet for doctors, identifying the reason behind constant fatigue is not always straightforward.

Medical experts told The Mail on Sunday that most cases of ongoing tiredness tend to fall into 12 broad categories, each with its own warning signs. But first, what does it actually mean to be “tired all the time”?

“This is not simply the normal weariness you feel after a demanding day at work,” explains Dr Dean Eggitt, a GP based in Doncaster.

“It is fatigue that should not be there — the kind that remains even after a full night’s sleep or a quiet weekend of rest. That sort of tiredness can be a signal that something is wrong. The important question is: what is making you unwell?” Here, specialists outline the possible reasons behind constant tiredness and fatigue.

Mental health

Experts say depression is one of the leading causes of feeling exhausted all the time. Although it is most commonly associated with a persistently low mood, fatigue is also a well-recognised symptom of the condition.

Research published in 2016, which reviewed findings from 26 separate studies, found that one in five patients who complained of constant tiredness were also experiencing depression. However, doctors would usually expect other symptoms to appear alongside fatigue.

“They may also struggle with sleep problems or poor self-esteem,” says Dr Eggitt.

‘They might also have little desire to do the things that usually make them happy.’

Experts add that antidepressants can also trigger fatigue.

Snoring

Sleep apnoea is a condition in which the throat muscles intermittently relax and collapse during sleep, blocking the airway and causing pauses in breathing. These episodes trigger snoring but they also disrupt sleep, leaving many sufferers constantly exhausted. 

‘If you sleep alone, you may not realise you are snoring,’ says Dr Eggitt. ‘And if you have a partner, they might not know that snoring is caused by sleep apnoea. So many patients live with this condition for years, which can lead to fatigue.’

The most common medical treatment is a continuous positive airway pressure (CPAP) machine, a mask which uses air pressure to keep the airways open. Patients can also improve their breathing and sleep by losing weight.

Lack of green vegetables

Vitamin deficiencies are often the reason why patients feel tired all the time and the most common, say experts, is low levels of iron. Iron is an essential mineral which carries oxygen throughout the body and is found in foods such as red meat, poultry and fish, as well as beans, lentils and spinach. Extremely low levels can lead to anaemia, where patients develop pale or yellowish skin, shortness of breath and fatigue. But even a mild deficiency can cause fatigue.

A third of adult women are affected compared to just 3 per cent of men because heavy periods can trigger a deficiency.

‘It’s one of the first things I ask women who tell me they are tired all the time: “Are you experiencing heavy periods?”,’ says Professor Dame Clare Gerada, a former president of the Royal College of General Practitioners. ‘If the answer is yes, then it’s most likely to be an iron deficiency, though I’d run a blood test to make sure. Another symptom of an iron deficiency is hair loss.’

Vegan or over 60

Another deficiency linked to fatigue is vitamin B12. This is mostly found in animal products such as meat and dairy and is crucial for making red blood cells and regulating the nervous system.

About a tenth of the six million vegans are deficient in B12, as are a fifth of people aged over 60 since the body needs more B12 as we get older. Symptoms develop gradually, with tiredness among the earliest, along with headaches, indigestion and difficulties with concentration. Over time, a deficiency can lead to nerve damage and psychosis.

Experts also flag up deficiencies in folate, which helps the body to repair DNA and make red blood cells. It is found in dark leafy greens, legumes and fortified breakfast cereals.

‘Vitamin B12 deficiency is less common than iron and a folate deficiency is even less common,’ says Dr Eggitt. ‘But once the more obvious causes like depression are ruled out, most GPs would likely order a blood test for these deficiencies.’

Low vitamin D levels can also lead to fatigue. The vitamin is mainly gained via sunlight – and oily fish in smaller amounts. Around one in six British adults are vitamin D deficient, which is why the NHS recommends taking a daily supplement between October and March.

Ultra-processed diet

Poor diet could be to blame for many cases of tiredness, say experts, and a Western diet has been strongly linked to fatigue.

About 60 per cent of the average British adult’s diet is made up of ultra-processed food, which has been linked to obesity, heart disease, cancer and dementia.

‘Our diet in the West is high in sugar, salt and saturated fats and this means many of us are not getting the right nutrients,’ says Dr Piril Hepsomali, a nutrition and psychology expert at the University of Reading. 

‘This goes beyond vitamin deficiencies like iron or B12. For instance, if you eat a cheap meal-deal sandwich or microwave meal from a supermarket, this is going to give you a short-term energy spike but then that’s followed by a energy reduction, which will leave you feeling tired. And over time, this diet raises inflammation in the body.’

Experts recommend eating more fruit, vegetables and seafood instead. ‘These foods contain nutrients that have been shown to improve energy levels,’ says Dr Hepsomali. ‘These include magnesium, vitamin D and fibre.’

Not enough water

Poor hydration is also to blame for fatigue problems.

The NHS recommends drinking six to eight glasses of water a day. However, a 2026 study in the UK found that about 40 per cent of the population don’t drink enough.

One in five were classed as dehydrated and men were less likely to drink enough water. ‘When you are dehydrated, your brain function is affected,’ says Dr Eggitt. ‘Inevitably, this will mean you feel tired all the time.’ Dehydrated patients would usually suffer from headaches as well as fatigue. Dr Eggitt adds that overconsumption of caffeinated drinks makes this issue even worse. ‘Caffeine leaves you dehydrated,’ he says. ‘And many Britons consume these drinks instead of water.’

Hormone issues

Many cases of tiredness are due to hormone issues. This is often connected to the thyroid, a butterfly-shaped organ in the neck which produces key hormones.

When it doesn’t produce enough of these hormones – hypothyroidism – this can lead to fatigue.

The condition affects about 2.3million people in the UK and is more common in those aged over 70 and women with a family history of the disease. It typically occurs when the immune system mistakenly attacks the thyroid. However, it can also stem from surgery, radiotherapy or side effects from medication.

Thankfully, experts say the disease is easy to diagnose and treat.

‘There’s a thyroid function blood test that can spot the condition fairly quickly,’ says Professor Ashley Grossman, a hormone expert at the University of Oxford.

‘And thyroxine [a daily hormone tablet] can fix the problem in about 95 per cent of cases.’

Sugar levels too high

Fatigue which comes on suddenly can often be due to diabetes, experts say. Most sufferers have type 2 diabetes which is caused by a lack of exercise or poor diet. But about a tenth have type 1, an autoimmune condition. In both types, the body doesn’t produce enough insulin to bring down blood sugar, which over time can lead to blindness, organ damage and death.

In the UK, type 2 diabetes affects over four million people and this figure is rising due to obesity. ‘Diabetes is one of the biggest causes of fatigue,’ says Dr Eggitt. ‘Other signs include needing to pee all the time and an urge to drink water constantly.’

Painful joints

Tiredness can be an early sign of a painful chronic joint condition, experts say. Inflammatory arthritis, where the immune system mistakenly attacks healthy joint tissue, affects about one million people in the UK. It differs from the more common osteoarthritis, which is caused by wear-and-tear, typically in old age.

Inflammatory arthritis includes axial spondyloarthritis, where damage mainly occurs in the spine, and psoriatic arthritis, where joint pain comes with a red skin rash. Crucially, they’re often accompanied by fatigue, due to the overactive immune system.

‘Silent killer’ 

Another condition that can trigger fatigue is kidney disease, although the symptom only tends to arise in its later stages. Chronic kidney disease occurs when the organs, which filter toxins from the blood, slowly stop working.

It affects about 7.2million adults in the UK and contributes to about 45,000 deaths a year. Crucially, it is symptomless in its earliest stages. ‘A huge amount of us have this disease without knowing it,’ says Dr Eggitt.

‘It’s slow-moving so, for most, it won’t be a problem, but for others it can be life-threatening.’

By the time symptoms begin, such as itchy skin and fatigue, the organs are usually beginning to fail and a transplant may be necessary. People with high blood pressure, diabetes and obesity are most at risk.

Fatigue that hit Cher

Perhaps the hardest cause of tiredness to diagnose – and treat – is chronic fatigue syndrome.

This rare condition, affecting about 250,000 people in the UK, causes extreme fatigue lasting six months or more. Experts believe it is often triggered by stress, a lack of sleep or an infection.

Singer Cher, 80, developed CFS in the 1980s after contracting the Epstein-Barr virus, forcing her to take a break for two years.

There is no test for the condition, so the only way to diagnose it is to rule out other causes first.

‘Most patients will wait years for a diagnosis,’ says Professor Dmitry Pshezhetskiy, a CFS expert at Norwich Medical School.

There is also no treatment so specialists help patients to manage symptoms. Professor Pshezhetskiy says: ‘In every case, the trick is to make sure patients don’t overexert themselves because this can make the symptoms worse.’

Cancer

It is everyone’s worst fear: fatigue that turns out to be cancer. Experts say that while cancer is rare, tiredness is an all-too common symptom of the disease.

‘When tumours begin to grow in the body, this takes up stores of energy, which is why patients often report feeling tired all the time,’ says Dr Eggitt.

For some patients, fatigue can be the only early sign of cancer. But for most there are other signs such as lumps, unexplained weight loss, change in bowel or bladder habits, bloating, a persistent cough or unusual bleeding.

Patients who feel tired all the time are unlikely to have cancer, but it’s important to check for the disease, which kills 167,000 every year in the UK.

Taking vitamin D supplements helped put me back in the saddle

When Rachelle Smith, 40, a horse-riding stunt performer from Wales, began feeling so exhausted after work that she could do little more than collapse onto the sofa, she assumed she was going early menopause. 

Once someone who filled her free time with sport and the gym, she found even coffee dates and riding with friends became too much, and she regularly cancelled plans at the last minute. 

‘I lacked motivation. I didn’t want to do anything – it was like my entire personality had changed,’ she says. 

She began napping during the day, something completely out of character, and structured her days around simply getting home and going to sleep. Aching joints, irregular periods, poor concentration and memory lapses followed, and she started turning down work as she was exhausted. 

Rachelle’s mother had gone through early menopause around the same age, so Rachelle initially believed the same was happening to her.

However, a private blood test, taken on a recommendation, revealed a different cause: she was severely deficient in iron and vitamin D. She began taking iron and vitamin D supplements as oral sprays. 

She also started eating more vitamin C-rich foods, such as red peppers and oranges, which studies show help the body absorb iron more effectively. In two weeks, her energy returned.

‘The fog lifted,’ she says. ‘I started running and I even took up horse riding again after work – something I had not done in years.’