Usually obesity is the result of overeating, but in rare cases (about 1%) excess weight gain is a symptom of another disease.

These can include:

  * Hypothyroidism. This is a condition where the thyroid gland, located in the neck, produces too little thyroid hormone. Thyroid hormone regulates our metabolism. So too little hormone slows the metabolism and often causes weight gain. If your doctor suspects thyroid disease as a cause of your obesity, he or she may perform blood tests to check your hormone levels.
  * Cushing’s syndrome. This condition results when the adrenal glands (located on top of each kidney) produce an excess amount of a steroid hormone called cortisol. This leads to a build-up of fat in characteristic sites such as the face, upper back, and abdomen.
  * Depression. Some people with depression overeat, which can lead to obesity.

There are also certain inherited conditions and other diseases of the brain that can cause excess weight gain. 

Certain medications, notably anti-depressants, steroid medications, certain high blood pressure drugs, and seizure medications can also cause increased body weight.

A doctor can determine if any of these conditions are responsible for your weight gain.