Endogenous depression is caused by biological factors or genetic predisposition rather than an outside stimulus, as opposed to its reactive equivalent. It encompasses a potential for significant morbidity by influencing the outcomes of other, unrelated medical conditions or even self-inflicted injury, and mortality, usually by suicide. Furthermore, endogenous depression may cause patient alienation from peers and substance abuse. However, symptoms can be reduced by appropriate medication.
Patients suffering from endogenous depression may conceal psychiatric symptoms and may address a general physician instead of a psychiatrist with somatic complaints    like headaches, muscle aches, numbness or abdominal distress, for fear of being diagnosed with a mental illness, that would cast a stigma on them. They may also genuinely believe physical symptoms are the cause of their low mood and dysphoria, not the other way around   . Aside from the symptoms enumerated above, affected individuals exhibit anhedonia, irritability, persistent sadness or sense of worthlessness, asthenia, isolation, eating disorders leading to weight changes, lack of motivation, insomnia, and difficulty in concentrating and memorizing new information. Disinterest in sexual activities is also noticed. Patients may find it difficult to impose their opinion, express strong feelings and cope with casual activities. They are often insecure and feel helpless. Perceived quality of life decreases, for the individual and his or her caregivers. Agitation or lethargy or swings between these two states are also frequently encountered. In more advanced stages, hallucinations and suicidal ideation arise, in parallel with social isolation. Patients may also provoke self-injury. Depression may also be a symptom of other psychiatric diseases, such as schizophrenia. When associated with psychosis, depression may also signalize bipolar affective disorder, substance abuse or organic brain syndrome. The prognosis of associated illnesses is two times worse in a depressed patient.
Symptoms may vary among age groups. Even preschool children may be affected  . Children of all ages more often present with irritability or decline in school grades or even marasmus , whereas elders more often have confusion and somatic signs as dominant findings.
There is no specific physical trait that helps diagnose endogenous depression aside from the fact that the physician may notice decreased hygiene, decreased movement and reactivity or agitation. However, infection, hypothyroidism, Cushing's syndrome, Addison's disease, hypopituitarism and a number of tumors and neurologic organic disorders may cause endogenous depression and must be eliminated by physical examination and laboratory workup. Blood tests should include a complete blood cell count, blood alcohol and vitamin B12 levels, toxicology screening, thyroid-stimulating hormone levels, liver, and kidney function tests. Dexamethasone suppression test and cosyntropin stimulation test can overrule Cushing's syndrome and Addison's disease.
When a neurologic substrate is suspected based on the presence of neurologic deficits, imaging modalities such as computer tomography, magnetic resonance imaging, positron emission tomography and single-photon emission computed tomography scanning may prove useful; the latter demonstrating regional perfusion deficits in affected patients  .
Several depression tests exist. Some are based on patient's understanding of depression , some describe and inquire about symptoms . Depression scales, such as Zung self-rating depression scale, geriatric depression scale, patient health questionnaire, Beck depression inventory, center for epidemiologic studies-depression scale or Hamilton depression rating scale should be used. They are self-applied or the last one should be carried out in a clinical setting.