Losing weight without trying may indicate an underlying condition. Conditions that cause unexplained weight loss include overactive thyroid, inflammatory bowel disease, and depression.

A good rule of thumb is to see your doctor if you’ve lost a significant amount — more than 5% of your weight — within 6 to 12 months.

In addition, take note of any other symptoms to discuss with your doctor. Read on to learn what can cause unexplained weight loss.

Not all weight loss is serious. It can happen after a life changing or stressful event. However, unexplained or unintentional weight loss may indicate one of these medical conditions:

  • Muscle loss (muscle atrophy): This can lead to unexpected weight loss from loss of muscle, most commonly if you don’t use your muscles for a while. It’s most common in people who don’t exercise, work desk jobs, or are bedridden.
  • Hyperthyroidism: An overactive thyroid develops when your thyroid gland makes too much thyroid hormone. These hormones control many functions in the body, including metabolism. If your thyroid is overactive, you’ll quickly burn calories even if you have a good appetite.
  • Rheumatoid arthritis (RA): This causes your immune system to attack the lining of your joints, leading to inflammation that can speed up metabolism and reduce your overall weight.
  • Type 1 diabetes: Your immune system attacks cells in your pancreas that make insulin. Without insulin, your body can’t use glucose for energy. This causes high blood glucose and causes your body to burn fat and muscle for energy.
  • Depression: This mood disorder affects the same brain parts that control appetite. This can lead to poor appetite and, eventually, weight loss. In some people, depression may increase appetite. The symptoms vary from person to person.
  • Inflammatory bowel disease (IBD): The chronic inflammation of IBD puts your body in a catabolic state, which means it constantly uses energy. IBD also disrupts ghrelin, the hunger hormone, and leptin, the satiety hormone. This results in decreased appetite.
  • Chronic obstructive pulmonary disease (COPD): This progressive lung disease includes emphysema and chronic bronchitis. In later stages, COPD can cause weight loss because labored breathing burns many calories.
  • Endocarditis: This is inflammation of your heart’s inner lining or endocardium caused by germs —usually bacteria — that enter the bloodstream and collect in your heart. Most people with endocarditis have a fever. It may come along with a poor appetite. Elevated body temperature also increases metabolism and burns fat, causing weight loss.
  • Tuberculosis (TB): This contagious condition usually affects the lungs. Weight loss and decreased appetite are major symptoms of TB, but the reasons aren’t fully understood.
  • Cancer: This is the general term for diseases that cause abnormal cells to quickly divide and spread. According to the American Cancer Society, one of the first signs may be unexplained weight loss of 10 pounds (lb) or more. Cancer increases inflammation, promoting muscle wasting and disrupting appetite-regulating hormones. A growing tumor may also increase your resting energy expenditure (REE) or how much energy your body burns at rest.
  • Addison’s disease: This rare condition causes the immune system to attack the adrenal glands. The adrenal glands can’t make enough hormones like cortisol and aldosterone. Cortisol regulates many functions, including metabolism and appetite. Low levels of cortisol may lead to poor appetite.
  • HIV: This virus attacks immune cells called T cells, making fighting infections difficult. If left untreated, HIV can lead to acquired immunodeficiency syndrome (AIDS). Advanced forms of these conditions often cause weight loss from nutritional deficiency and decreased hunger caused by illness symptoms. Learn more about HIV and weight loss.
  • Congestive heart failure (CHF): This develops when the heart can’t fill up with enough blood, pump blood with enough force, or both. Your digestive system can’t receive enough blood, leading to nausea and early fullness. It might be hard to breathe while eating, and the inflammation in damaged heart tissue also speeds up metabolism.

A few other factors may make it more likely to lose weight without an explanation. These include:

In older adults

Research shows that unexplained weight loss occurs in 15–20% of people ages 65 and above.

Smoking and more excellent body fat distribution also increase the chance of unexplained weight loss in older adults.

In children and adolescents

The following risk factors increase the chance of unexplained or unintentional weight loss in children:

  • Poverty: In the U.S., living in poverty is the biggest risk factor for failure to thrive, which is a child below the standard height and weight values for their age.
  • Challenges with breastfeeding to chestfeeding: It’s common to have difficulty with nursing. Even if you’re using formula, as a new parent, you may need time to learn how to prepare it correctly for your baby. It’s common to make mistakes in the beginning. That said, watch your baby’s weight, consult your pediatrician, and ask for help when needed. Learn what to do if your baby won’t nurse and how to increase your baby’s weight.
  • Allergies: A baby can also develop an allergy to a particular formula, which may take time to identify.
  • Eating disorders: The prevalence of eating disorders in teens is about 2.7%, with females having a higher chance of developing it than males. It’s common for teens to hide their disorder, but one sign can be unexplained weight loss. It doesn’t necessarily mean they have an eating disorder, but it’s a sign they may need an evaluation. Learn about the signs of an eating disorder in teens.

[the terms “male” and “female”]

In this article, we use “male and female” to refer to someone’s sex as determined by their chromosomes and “men and women” when referring to their gender (unless quoting from sources using nonspecific language).

In males vs. females

Compared with females, males have a higher rate of:

Females are at a higher chance of developing COPD. They are also 2 to 10 times more likely to develop hyperthyroidism and 2 to 3 times more likely to have RA.

Since there are many potential causes of unexplained weight loss, the diagnosis will probably start with a thorough evaluation. Your doctor will ask about your symptoms and history of known medical conditions or medications.

In a 2017 study, researchers asked subjects to submit various blood test samples and do a chest x-ray.

Older adult patients with no other symptoms were asked to undergo a dental exam, a barium swallow test, and a psychological assessment for depression.

In cases where the doctors couldn’t find an apparent cause, they continued to follow the patients.

That said, unexplained weight loss isn’t always recognized by doctors. In a 2023 study, researchers found doctors recognized this as a symptom in only 21% of cases.

The specific treatment for unexplained weight loss will depend on the underlying cause. Research also shows that a quarter of people with unexplained weight loss don’t get a diagnosis even after a comprehensive medical exam.

In cases where the weight loss is severe enough that the doctor suspects nutritional deficiency, they may ask you to adjust your diet or consume nutritional supplements even if an exact cause hasn’t yet been established.

What is the most common cause of unexplained weight loss?

According to a 2017 study of 2677 patients with unexplained weight loss in a Barcelona hospital, 33% of the subjects were diagnosed with some type of cancer, 37% had noncancerous underlying causes, and 16% were diagnosed with a mood disorder. In patients over 65, oral disorders were the second biggest cause after cancer.

Which cancer causes weight loss?

In the same 2017 study, the majority of those diagnosed with cancer were found to have pancreatic cancer and lymphoma. After that, kidney, bladder, and ureteral cancers were most common.

When should I be worried about unexplained weight loss?

It’s common for your body weight to fluctuate. However, something else might be happening if you are losing weight without changing your habits. If you experience a 5% weight loss in 6 to 12 months or notice any of the above symptoms, visit your doctor.

If you’ve been losing weight without conscious effort, it isn’t necessarily something to worry about but could be a sign of an underlying medical condition.

See your doctor if your weight loss seems significant, especially if you have additional symptoms.