Psychoneuroendocrinology is the clinical study of hormone fluctuations and their relationship to human behavior. It may be viewed from the perspective of psychiatry, where in certain mood disorders, there are associated neuroendocrine or hormonal changes affecting the brain. It may also be viewed from the perspective of endocrinology, where certain endocrine disorders can be associated with psychiatric illness. Brain dysfunctions such as in the hypothalamus can affect the endocrine system, which in turn can result in psychiatric symptoms. This complex blend of psychiatry, neurology and endocrinology is needed to comprehensively understand and treat psychiatric illnesses.