Unexplained weight loss may be a cause for concern when it happens rapidly over a short period of time. In general, it is considered normal for a person to lose weight during times of stress, like moving, starting a new job, ending a relationship, or experiencing the loss of a family member.

Rapid weight loss that is not explained by these factors or does not occur as the result of a diet or increased levels of physical activity could indicate the presence of a health problem like thyroid disease, uncontrolled diabetes, or cancer.

For this reason, it is important to consult a primary care provider for any episodes of unexplained weight loss. A workup may include diagnostic tests to identify the cause of the unintentional weight loss and guide appropriate treatment.

Possible causes

Unexplained weight loss may be due to:

Gastrointestinal (GI) disorders, such as Crohn's disease, ulcers, ulcerative colitis (UC), chronic pancreatitis, mesenteric ischemia, or celiac disease;
Neurological conditions, such as Parkinson's disease, dementia, multiple sclerosis, or stroke;
Endocrine disorders, such as hyperthyroidism, adrenal insufficiency, or uncontrolled diabetes;
Lung diseases, such as emphysema, chronic bronchitis, or chronic obstructive pulmonary disease (COPD);
Infectious diseases, such as tuberculosis or HIV;
Heart disease, such as advanced cardiac insufficiency;
Mental health conditions, such as depression, anxiety, anorexia, bulimia, bipolar disorder, obsessive-compulsive disorder (OCD), or schizophrenia;
Side effects of medications like antiepileptics (seizure drugs), antidepressants, anxiolytics (anxiety medications), diuretics, or laxatives;
Cancer, with weight loss most commonly noted in cases of multiple myeloma, leukemia, or lymphoma.

In addition, excessive use of alcohol or drugs can also result in rapid unintentional weight loss.

Unintentional weight loss can also be caused by specific factors depending on age and phase of life:

1. Weight loss in the elderly

Gradual weight loss can be a normal part of aging attributed to lack of appetite, changes in taste, or medication side effects. Another common cause of weight loss in the elderly is dementia, which can make a person forget to eat.

Loss of muscle mass is also considered a normal part of aging in people over 50, primarily due to a reduction in muscle fibers, decreased production of hormones like estrogen and testosterone, and a decrease in physical activity. This progressive loss of muscle mass due to aging is also known as sarcopenia.

Older adults may also experience a loss of bone mass that makes the bones more fragile and increases the risk for fractures.

2. Weight loss during pregnancy

Weight loss during pregnancy is not considered normal, but may occur due to nausea and vomiting in the first trimester that makes it difficult to maintain an adequate diet. Hyperemesis gravidarum (HG) is a severe form of nausea and vomiting during pregnancy that can result in significant weight loss.

In these situations, the pregnant person should be followed by an OBGYN and a registered dietitian to help prevent complications like growth restriction in the fetus. As a general rule, healthy weight gain for people with a BMI in the normal range is 25 to 35 lb (10 to 15 kg) during the whole pregnancy.

3. Weight loss in babies

Weight loss is common in newborns, who may lose up to 10% of their body weight in the first days of life due to factors like fluid loss and feeding adjustment.

Beyond this period, babies are expected to gain about 4 to 7 ounces per week for the first several months of life and remain on a consistent growth curve for weight and height as they get older.

Babies with slow weight gain or weight loss need to be monitored closely by a pediatrician to avoid developmental delays and other complications.

What to do

Unexplained weight loss should be evaluated by a doctor or other healthcare provider to determine an underlying cause and arrange for the appropriate treatment.

To diagnose the cause of unintentional weight loss, a provider will first perform an evaluation of symptoms and order tests based on their suspicions, such as blood, urine, and stool tests. Exams like an MRI (magnetic resonance imaging) or chest X-ray may also be ordered based on the results of these tests.

This initial workup is typically done with a primary care provider or general practitioner, followed by a referral to specialists like an endocrinologist, psychiatrist, or oncologist based on the findings.

When to worry

Unexplained weight loss is cause for concern when it exceeds 5% of a person's normal body weight over a period of 6 to 12 months.

It is also important to pay attention to symptoms like fatigue, loss of appetite, changes in bowel habits, bone pain, swollen lymph nodes, and frequent infections like colds. In these cases, it is advised to consult with a primary care provider or general practitioner for a work up in order to prevent complications.

Unexplained Weight Loss: 9 Causes and When to Worry