IGF1 and major depressive disorder: It is a 70-amino acid peptide and is mainly produced in the central nervous system (CNS) and peripheral tissues such as the liver.4,5 IGF-1 is mainly regulated via growth hormone secretion.6 In addition, the secretion of IGF-1 has been shown to affect the hypothalamus-pituitary-adrenal axis reciprocally, which is believed to be one of the pathophysiologies of MDD.7 There are four main reasons why researchers have focused on the relationship between IGF-1 and major affective disorders.