Blood tests for unexplained weight loss: what a GP may check and why

The NHS describes unintentional weight loss as losing weight without changing your diet or exercise routine, and says that it can be a sign of stress or a serious illness. A GP will ask about your health and may arrange some tests or scans, and depending on the possible cause may advise on what you can do, recommend treatment for a condition that is causing the weight loss, or refer you to a specialist.

Because the possible causes are so varied, the tests are chosen to match. The honest summary is that a GP typically combines a careful history and examination with a small number of blood tests that screen for the more common treatable causes, followed by further tests directed by what is found. The sections below describe the tests that NHS and NICE sources link to weight loss.

Not everyone will have the same tests. Weight loss with abdominal pain in someone aged 40 or over leads down a different route from weight loss with a racing heart and tremor. A test being listed here does not mean it is done in every case.

nidirect, adapting NHS content, says body weight can fluctuate regularly, but persistent, unintentional loss of more than 5 per cent of body weight over 6 to 12 months is usually a cause for concern, and may be a sign of malnutrition. It lists associated symptoms including tiredness, loss of appetite, a change in toilet habits and an increase in illnesses or infections.

NICE's suspected cancer guideline NG12, updated in 2026, uses a threshold of more than 5% mean weight loss within a 6-month period for people aged 60 and over. For these people, it recommends carrying out an assessment for additional symptoms, signs or findings that may help clarify which cancer is most likely, and offering urgent investigation, a suspected cancer pathway referral or a non-specific symptoms pathway referral.

The NHS and nidirect list a wide range of causes of unintentional weight loss. The table matches the main causes to the test that the NHS describes for each. It is a map of reasons, not a prescribed order of testing.

• Full blood count: counts red cells, white cells and platelets. The NHS uses it to look for anaemia, and NICE uses it when blood cancers are suspected.

• Thyroid function: a blood test showing whether the thyroid is overactive, which can cause weight loss with tiredness, trembling, palpitations and sensitivity to heat.

• Blood glucose or HbA1c: looks for diabetes, which can cause weight loss with thirst, tiredness and passing more urine.

• Coeliac antibodies: look for coeliac disease, which can cause weight loss with bloating, diarrhoea or anaemia.

• Calcium, liver and kidney tests: nidirect lists heart, kidney, lung and liver disease among less common causes of weight loss, and NICE includes calcium in the tests for suspected myeloma.

• Inflammatory markers: NICE lists plasma viscosity or ESR in the myeloma work-up; these are not specific to any one cause.

Blood tests are often only the first step. NICE guideline NG12 says to offer quantitative faecal immunochemical testing (FIT) to guide referral for suspected colorectal cancer in adults aged 40 and over with unexplained weight loss and abdominal pain, adults under 50 with rectal bleeding and weight loss, and adults aged 50 and over with weight loss, among other combinations.

It also says to offer an urgent direct access chest X-ray for people aged 40 and over with unexplained weight loss if they have ever smoked, or with other unexplained symptoms such as cough, fatigue, shortness of breath or chest pain. For people aged 55 and over with weight loss and upper abdominal pain, reflux or dyspepsia, it advises a suspected cancer pathway referral for oesophageal or stomach cancer.

For women, and trans men and non-binary people with female reproductive organs, who report unexplained weight loss, fatigue or changes in bowel habit, NG12 says to consider carrying out tests in primary care, which include CA125 measurement for those aged 40 or over with persistent symptoms that suggest ovarian cancer.

The NHS says you should see a GP if you keep losing weight without changing your diet or exercise routine, and as soon as possible if you have weight loss and other symptoms. A note of when the weight loss began, how much you have lost, any change in appetite, bowel habit, thirst or energy, and your medicines will help, as the NHS lists side effects of some medicines, such as levothyroxine, among causes.

A normal blood test does not close the question if you are still losing weight. NICE guideline NG12 is designed to be used alongside the full picture, and the NHS says that the earlier the cause is found, the sooner it can be treated. If weight loss continues, tell the GP so the plan can be reviewed.