What Can Cause Unexpected Weight Loss

Unintentional or unexpected weight loss occurs when there is a sudden and noticeable weight loss without the intent to lose weight. It can be caused by a number of factors, both physical and psychological. Whatever the reason, rapid weight loss can be a big health concern¹ and should not be ignored.

Unexpected weight loss is one of the reasons why doctors routinely weigh patients even during routine physical check-ups. It is definitely something to watch for.

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Possible reasons for losing weight unexpectedly

While there are potentially a number of reasons for unexpected weight loss, some are more common than others.

Here are some possible causes for suddenly losing weight in a short period of time.

Major short-term stressors

The most common cause of unintentional weight loss is experiencing a stressful life event. It is typical to noticeably lose weight after the death of a loved one, divorce, losing or changing jobs, and moving house, among others.

In these cases, your weight will typically return to normal when the situation that’s stressing you out has been resolved.

Chronic stress

Chronic stress can cause longer-term weight loss for many of the same reasons. It can affect your metabolism (in both directions), so stress can result in both weight loss and weight gain, depending on how your body responds.

Eating disorders

Any form of eating disorders can cause weight loss.² While anorexia nervosa is most known for causing it, sudden weight loss can also be a result of bulimia and avoidant-restrictive food intake disorder (ARFID).³ With severe ARFID, you are struggling to find enough to eat or to balance your diet within your identified restrictions.

If you suspect you have an eating disorder, it is crucial to seek help. Eating disorders are treatable and you can get your weight back to normal. More importantly, getting help will improve your overall physical and mental health.


Depression suppresses your appetite, which leads to weight loss.⁴ Some people with depression lose interest in their favorite food and lack the desire to eat in general. They may not even have the energy to get up, much less to prepare food.

If you are depressed and are struggling to manage your food intake, reach out and ask for help. While you can just ask someone for help with food shopping and preparation, you still need the energy and desire to eat, and only you can help yourself with that.

Therefore, seeing a mental health professional when you admit you might have depression can go a long way in helping you deal with these depressive symptoms.


Severe anxiety can cause you to lose weight because when you are anxious all the time, you will find it hard to eat.  However, it may also be because the molecule in the brain that’s causing anxiety also increases your metabolism and how much you burn fat.⁵

Because of this, people with anxiety disorders are more likely to lose weight fast; this may also impact people under constant stress.

Overactive thyroid

Your thyroid plays a key role in regulating your metabolism. An overactive thyroid (hyperthyroidism) can result in weight loss while an underactive thyroid (hypothyroidism) leads to weight gain.

Hyperthyroidism is caused by various conditions. The most common treatments are medication, radioactive infusion, or surgery, depending on why it is overactive.⁶

Some people known to have hypothyroidism may also experience sudden weight loss.⁶ This may be an indicator that the amount of hormone being produced by your thyroid has increased, as can happen in cases of thyroiditis (inflammation of the thyroid gland).

Unexpected weight loss, while you are on medication for hypothyroidism, may mean your replacement hormone dosage is too high. If this occurs, visit your doctor to get your blood checked and see if your dosage needs to be adjusted.⁶


Weight loss is often the first sign of cancer.⁷ However, this does not mean that most unexpected weight loss is caused by cancer.

While 40% of people with cancer reported unintended weight loss as the first symptom, it is far more likely that your weight loss is being caused by something more treatable, such as anxiety or thyroid issues.

Drugs or alcohol

Drugs, both prescribed and illicit, and alcohol abuse can cause weight loss.⁸ Some illicit drugs suppress your appetite while medications may have weight loss as a side effect.

If you are on medication (new or long-term) to treat a chronic condition, check to see if weight loss is a potential side effect. If it is, talk to your doctor about mitigating the weight loss or changing your meds.

Oral and dental problems

Problems with your mouth can make it difficult for you to eat due to injury or pain. This can cause weight loss.⁹ Typical oral health issues include losing teeth, a new orthodontic device, mouth ulcers, or jaw pain.

Your dentist can provide advice if your dental issues are making it hard for you to eat.

Gut and digestive problems

Celiac disease can cause extreme weight loss and malnutrition, and the only cure is to remove all gluten from your diet.¹⁰ You may also need to replace lost vitamins and nutrients to see an improvement in your health.

Other issues with your stomach and gut can also lead to weight loss, including ulcerative colitis and  Crohn’s disease.¹¹

These issues cause weight loss directly by decreasing the amount of nutrients you can absorb or indirectly by suppressing your appetite. When weight loss is accompanied by constipation and/or diarrhea, nausea, and appetite reduction, it’s likely to be an issue with your gut.


Dysphagia is difficulty swallowing, which is a symptom of a condition, such as acid reflux, esophagitis, nervous system disorders, or esophageal cancer.¹²

Dysphagia is managed by treating the underlying condition. Some people with dementia also develop dysphagia and may require a special diet.

Other issues that may cause unexpected weight loss

Unexpected weight loss can be caused by a huge variety of conditions; so many that it isn’t practical to cover them all here.

However, the following warrant a special mention here:

  • Chronic inflammatory diseases (such as lupus¹³)

  • Infections (such as TB¹⁴ or HIV¹⁵), and 

  • Dementia.¹⁶

In most cases, the weight loss due to the above conditions is an identifiable symptom of a known underlying problem. It is then treated as part of overall case management.

When should you worry about weight loss?

You should consult your doctor about weight loss if it is unintentional.¹⁷ It is especially important if:

  • You can’t pinpoint a cause.

  • You have lost more than 5% of your weight in under a year.

  • You are also experiencing fatigue, loss of appetite, a change in toilet habits, and/or an increase in illnesses or infections.

In these cases, your doctor will need to investigate to establish the reason for your symptoms.

The lowdown on unexpected weight loss

Unexpected and unintentional weight loss can be caused by a variety of things, which range from stressful life events to cancer. The most common causes are stress, depression or anxiety, an eating disorder, or thyroid issues.

If you have experienced unexpected weight loss, especially if you have not also experienced a stressful event, talk to your doctor. Together, you can identify the potential causes and create a treatment plan to reverse your unintended weight loss and bring you back to your normal weight.

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