Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and physical well-being. It may include feelings of sadness, anxiety, emptiness, hopelessness, worthlessness, guilt, irritability, or restlessness.
Depressed people may lose interest in activities that once were pleasurable, or suffer cognitive impairments (e.g., difficulty concentrating, remembering details, making decisions). They may contemplate or attempt suicide. Their weight may change dramatically. Insomnia, excessive sleeping, change in sleep patterns (e.g., waking in the middle of the night or early in the morning and being unable to fall asleep again), fatigue, loss of energy, and aches, pains or digestive problems that are resistant to treatment may be present.
Depressed mood is a normal reaction to certain life events, a symptom of many medical conditions (e.g., Addison's disease, hypothyroidism), and a feature of certain psychiatric syndromes.
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 completed suicides the individuals had seen a physician within the prior year before their death, 45%-66% within the prior month. Approximately 33% - 41% of those who completed suicide had contact with mental health services in the prior year, 20% within the prior 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 50% 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. She was however told by a neurologist that it was benign. After a worsening of symptoms, upon the second opinion of another neurologist, the tumor was removed. After the surgery she no longer suffered from "depression".