Why Am I Losing Weight and Feeling Tired? Common Causes

Unexplained weight loss and fatigue often occur together when the body's metabolic balance is disrupted. Fatigue is a persistent exhaustion, while significant weight loss is usually defined as losing 5% of body mass unintentionally over 6-1

Medically reviewed by Dr. Roshan Basnet, MBBS, MD (Internal Medicine) on 2026-08-25. See the medical review policy.

Introduction

When to get emergency help

Call an ambulance or go to your nearest emergency department now if you have a sudden severe headache, chest pain, trouble breathing, coughing up blood, black or bloody stools, a stiff neck with fever, a rash that does not fade when pressed, slurred speech, one-sided weakness, confusion, a seizure, fainting, or thoughts of harming yourself.

Experiencing unexplained weight loss alongside persistent fatigue often signals an underlying shift in the body's metabolic or physiological state. Everyone feels tired occasionally, and minor weight fluctuations happen due to diet changes. But when these two issues occur together—especially unintentionally—it may warrant a closer look by a medical professional.

Fatigue in this context is a deep exhaustion that does not improve with rest. Significant weight loss is typically defined as losing more than 5% of body weight within six to twelve months without trying. These symptoms are non-specific, meaning they could point to a wide variety of conditions ranging from psychological stress to chronic physical illnesses.

The body requires a delicate balance of nutrients and hormones to maintain energy levels and body mass. When an underlying condition disrupts this balance, the body may begin to consume its own fat and muscle stores for energy. This leads to weight loss, while the effort of fighting an illness or dealing with a metabolic imbalance can cause profound lethargy.

Common Causes

• Hyperthyroidism

  An overactive thyroid gland may accelerate the body's metabolism significantly. When the thyroid produces too much thyroxine, the body's functions speed up. This could lead to rapid weight loss even if appetite increases. The metabolic strain often leaves individuals feeling physically exhausted and mentally drained. Learn more about hyperthyroidism.

• Type 2 Diabetes

  In cases of undiagnosed or poorly managed diabetes, the body may be unable to properly use glucose for energy. Instead, the body might start burning fat and muscle for fuel, leading to weight loss. Because the cells are not receiving the energy they need, persistent fatigue is a very common side effect. Read about type 2 diabetes.

• Depression and Anxiety

  Mental health conditions can have profound physical manifestations. Depression may lead to a loss of appetite and a lack of energy or motivation. Chronic anxiety can keep the body in a constant state of 'fight or flight,' burning through energy reserves and leading to physical depletion. See our guide on depression.

• Digestive Disorders

  Conditions such as Celiac disease, Crohn’s disease, or ulcerative colitis may interfere with the body's ability to absorb essential nutrients. This malabsorption can cause the body to lose weight despite eating normally. The lack of nutrients often results in chronic fatigue and anemia.

• Chronic Obstructive Pulmonary Disease (COPD)

  For those with advanced lung issues, the simple act of breathing requires significantly more energy than it does for a healthy person. This increased caloric demand, combined with the fatigue caused by lower oxygen levels, often results in weight loss over time.

• Rheumatoid Arthritis

  As an autoimmune condition, rheumatoid arthritis can cause systemic inflammation. This inflammation may speed up the metabolism and increase energy expenditure. The chronic pain and immune response frequently lead to severe exhaustion.

Less Common Causes

• Adrenal Insufficiency (Addison's Disease)

  This occurs when the adrenal glands do not produce enough certain hormones, like cortisol. This deficiency can affect appetite and how the body handles stress and energy, potentially leading to weight loss and extreme tiredness.

• Chronic Infections

  Conditions like tuberculosis, endocarditis, or HIV/AIDS may place a heavy burden on the immune system. The body uses a vast amount of energy to fight these long-term infections, which could lead to a 'wasting' effect and persistent lethargy.

• Malignancy (Cancer)

  Certain types of cancer may increase inflammation and change the way the body uses calories. While not the most common cause for everyone, unexplained weight loss is sometimes one of the first visible signs of an underlying oncological process.

• Pancreatitis

  Chronic inflammation of the pancreas can impair the production of enzymes needed for digestion. This may lead to malabsorption of fats and nutrients, causing weight to drop and energy levels to plummet.

Associated Symptoms

When weight loss and fatigue occur together, they are rarely the only symptoms present. Depending on the underlying cause, individuals might also experience digestive changes such as chronic diarrhea, constipation, or abdominal pain. If the cause is hormonal, symptoms might include heat or cold intolerance, tremors, or palpitations. Skin changes, such as dryness or unusual darkening, can sometimes be associated with metabolic or adrenal issues. Additionally, many people report 'brain fog,' a lack of mental clarity, or changes in mood such as irritability or persistent sadness. Tracking these secondary symptoms can be vital for a healthcare provider to narrow down a potential diagnosis.

Diagnosis

To evaluate these symptoms, a healthcare provider will typically begin with a thorough medical history and physical examination. They may ask about the timeline of the weight loss, dietary habits, and the specific nature of the fatigue. Blood tests are a standard first step and may include a Complete Blood Count (CBC) to check for anemia or infection, a Comprehensive Metabolic Panel (CMP) to evaluate kidney and liver function, and a Thyroid Stimulating Hormone (TSH) test to check for thyroid dysfunction. Depending on the initial findings, a doctor might also order imaging tests like an X-ray or CT scan, or specialized tests such as a colonoscopy or endoscopy if digestive issues are suspected.

Treatment Options

Treatment for weight loss and fatigue is entirely dependent on the identified cause. If the symptoms are due to a nutritional deficiency, a healthcare provider might recommend dietary adjustments or specific supplementation. For conditions like diabetes or thyroid disorders, the primary goal is to stabilize hormone or glucose levels through lifestyle changes and medical management. If a mental health condition is identified, therapy or counseling may be recommended. In cases where an inflammatory or autoimmune condition is present, the focus is often on reducing systemic inflammation to allow the body to recover its energy and maintain a healthy weight.

Medicines Sometimes Used

Various medications may be prescribed by a doctor depending on the underlying diagnosis. For thyroid issues, medications to balance hormone levels might be used. If diabetes is the cause, insulin or other glucose-regulating medications may be prescribed by a doctor. In cases of depression or anxiety, antidepressants or anti-anxiety medications might be recommended. If the weight loss is linked to severe inflammation, a doctor might prescribe corticosteroids or other anti-inflammatory drugs. It is important to remember that these medications should only be taken as prescribed by a healthcare professional after a formal diagnosis has been made.

When to See a Doctor

It is generally advisable to consult a healthcare provider if you experience unintentional weight loss of more than 5% of your body weight within a few months. You should seek medical attention promptly if the fatigue and weight loss are accompanied by 'red flag' symptoms. These include persistent fever, night sweats, severe abdominal pain, significant changes in bowel habits, or a palpable lump or mass. Additionally, if you find that your fatigue is so severe that it prevents you from performing daily tasks or if you experience thoughts of self-harm, you should contact a medical professional or mental health service immediately.

This can be an emergency. If you have self-harm, call an ambulance or go to your nearest emergency department now. Do not wait for a doctor's appointment.

Frequently Asked Questions

This content is for educational purposes only and does not replace professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.

For related medicine and condition background, see Insulin.

This article is for informational purposes only and does not replace professional medical advice. Consult your healthcare provider before making medical decisions.

Educational information only. Always consult a qualified healthcare professional for medical advice.