BDNF and depressive disorder: Alterations in the expression of BDNF are relevant because of three reasons: 1) the BDNF promoter is responsive to thyroid hormones (133), 2) this neurotrophic factor plays a key role in the postmitotic phase of the neurogenic process, promoting the survival of newborn neurons (134, 135), and 3) several studies have highlighted associations between low levels of BDNF and the development of behavioral symptoms of depression (136).