As a rule, if you have lost weight and do not know why then see a doctor. A doctor's assessment and tests will usually be able to find the cause.
It is common to lose weight if you have a serious disease. For most serious diseases, there will usually be one or more other symptoms that develop well before any weight loss. The weight loss in these situations is explained and can often be expected.
However, occasionally, the first symptom to develop in some serious diseases is weight loss. Other symptoms usually develop at some point later. Also, some people with weight loss as a first symptom actually have one or more symptoms if they were questioned about them. But, they may not have been aware of the significance of the other symptoms. For example, weight loss is a common symptom of untreated type 2 diabetes. Some people may see a doctor to say that they have lost weight and don't know why. A doctor may then ask if they have been passing more urine than usual (a typical symptom of diabetes). To this question the person may say something like "now you mention it, yes I have. But I had just put that down to my ageing prostate gland and not thought much about it."
The rest of this article lists some of the more common conditions where weight loss is sometimes the first symptom noticed by the person, relative or friend. Obviously, it excludes people who are dieting or exercising to lose weight deliberately. And the list is not exhaustive.
In the vast majority of cases of cancer, other symptoms will develop before any weight loss. There are many symptoms that can occur, depending on the type and site of the cancer. However, occasionally, weight loss is the first symptom noticed. The weight loss is partly due to the increased activity of cancer cells in the body that require nutrition. (Weight loss in more advanced cancer is due to various combined factors.)
Type 2 diabetes occurs mainly in people aged over 40. The four common symptoms are: being thirsty a lot of the time, passing large amounts of urine, tiredness and weight loss. Some people pass off the first three symptoms to do with other things, or 'getting older'. So, weight loss is sometimes the first thing reported to a doctor. The weight loss is partly due to the sugar (glucose) that is passed out with the urine.
Depression has a number of symptoms. However, some people do not recognise that they have depression. They may feel low and not right, but cannot say why, and even think of it as just normal for them. Many people with depression can hide their true feelings from friends and family. Weight loss may develop due to not eating much. It is sometimes friends or family who may actually notice that a person has lost weight and it then turns out to be due to depression.
Some people who drink a lot of alcohol do not look after themselves very well. Their diet may not be very good and they may lose weight. However, they may be in denial about an alcohol problem. It may be friends or family who actually notice that the person has lost weight. Indeed, this is a way that a hidden alcohol problem in some people comes to light. Family and friends may show concern about weight loss and not be aware that the person has an alcohol problem.
Hyperthyroidism means that you make too much of a hormone called thyroxine. The extra thyroxine causes many of your body's functions to speed up. There are lots of possible symptoms such as being restless, nervous, emotional, irritable, sleeping poorly, tremor, sweating, diarrhoea and being 'always on the go'. Another typical symptom is losing weight despite an increased appetite. This may be a first symptom. But, again, some of the other symptoms that may occur such as being restless, nervous, emotional or irritable may just be put down to a stressful time of life or to your personality. In the early stages of hyperthyroidism some people actually feel well and full of energy and don't realise they have a problem, but lose weight.
It would be unusual not to have other symptoms with a chronic infection. But, occasionally, the first symptom that is noticed with infections such as tuberculosis or HIV is weight loss. Hookworm infection of the gut is another example where weight loss may be the first indication of disease. Hookworm is a very common problem worldwide (but rare n the UK). Likewise, there are various conditions that cause persistent inflammation of a part or parts of the body. Again, other symptoms would be usual. But, sometimes an inflammatory condition such as polymyalgia rheumatica may just make you feel unwell and lose weight prior to other symptoms developing.
There are various diseases that can affect the way food is absorbed and taken into the body from the gut. These would normally cause symptoms such as diarrhoea. However, weight loss may be the first symptom noticed.
People in the early stages of dementia may appear to get by reasonably well, but in fact they are not able to look after themselves very well. They may have a poor diet but appear happy in themselves. Again, it is sometimes friends or family who may first notice that a person is not right and has lost weight and it then turns out to be the early stages of dementia.
If you have lost weight and do not know why then see a doctor. This is because, as a general rule, the earlier a serious problem is diagnosed, the better the chance that treatment may improve the outlook (prognosis).
Your doctor is likely to ask various questions. He or she may actually uncover other symptoms that you were either unaware of or did not think were significant. He or she may also examine you. Following a doctor's assessment, the likely cause may be clear and your doctor will advise on what to do next. Tests may or may not be appropriate.
The treatment depends on the cause. See individual leaflets on the various diseases that can cause weight loss.