Depression, one of the most commonly diagnosed psychiatric disorders, is being diagnosed in increasing numbers in various segments of the population worldwide. Depression in the United States alone affects 17.6 million Americans each year or 1 in 6 people. Depressed patients are at increased risk of type 2 diabetes, cardiovascular disease and suicide. Within the next twenty years depression is expected to become the second leading cause of disability worldwide and the leading cause in high-income nations, including the United States. In approximately 75% of suicides, the individuals had seen a physician within the prior year before their death, 45–66% within the prior month. About a third of those who died by suicide had contact with mental health services in the prior year, a fifth within the preceding month.
There are many psychiatric and medical conditions that may mimic some or all of the symptoms of depression or may occur comorbid to it. A disorder either psychiatric or medical that shares symptoms and characteristics of another disorder, and may be the true cause of the presenting symptoms is known as a differential diagnosis.
Many psychiatric disorders such as depression are diagnosed by allied health professionals with little or no medical training, and are made on the basis of presenting symptoms without proper consideration of the underlying cause, adequate screening of differential diagnoses is often not conducted. According to one study, "non-medical mental health care providers may be at increased risk of not recognizing masked medical illnesses in their patients."
Misdiagnosis or missed diagnoses may lead to lack of treatment or ineffective and potentially harmful treatment which may worsen the underlying causative disorder. A conservative estimate is that 10% of all psychological symptoms may be due to medical reasons, with the results of one study suggesting that about half of individuals with a serious mental illness "have general medical conditions that are largely undiagnosed and untreated and may cause or exacerbate psychiatric symptoms".
In a case of misdiagnosed depression recounted in Newsweek, a writer received treatment for depression for years; during the last 10 years of her depression the symptoms worsened, resulting in multiple suicide attempts and psychiatric hospitalizations. When an MRI finally was performed, it showed the presence of a tumor. However, she was told by a neurologist that it was benign. After a worsening of symptoms, and upon the second opinion of another neurologist, the tumor was removed. After the surgery, she no longer had depressive symptoms.
Autoimmune disorders
Celiac disease; is an autoimmune disorder in which the body is unable to digest gluten which is found in various food grains, most notably wheat, and also rye and barley. Current research has shown its neuropsychiatric symptoms may manifest without the gastrointestinal symptoms.
"However, more recent studies have emphasized that a wider spectrum of neurologic syndromes may be the presenting extraintestinal manifestation of gluten sensitivity with or without intestinal pathology."
Lupus: Systemic lupus erythematosus (SLE), is a chronic autoimmune connective tissue disease that can affect any part of the body. Lupus can cause or worsen depression.
Bacterial-viral-parasitic infection
Lyme disease; is a bacterial infection caused by Borrelia burgdorferi, a spirochete bacterium transmitted by the Deer tick (Ixodes scapularis). Lyme disease is one of a group of diseases which have earned the name the "great imitator" for their propensity to mimic the symptoms of a wide variety of medical and neuropsychiatric disorders. Lyme disease is an underdiagnosed illness, partially as a result of the complexity and unreliability of serologic testing. "Because of the rapid rise of Lyme borreliosis nationwide and the need for antibiotic treatment to prevent severe neurologic damage, mental health professionals need to be aware of its possible psychiatric presentations.