PCSK9 and obesity due to melanocortin 4 receptor deficiency: Although studies have confirmed that alteration in the hypothalamic-pituitary-gonadal axis, insulin resistance (IR) as well as excessive accumulation of aromatase and estrogen is closed related to obesity-induced T deficiency [7, 8], recent progress reports that proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibits the uptake of cholesterol for T synthesis, meanwhile, its upregulation was significantly negatively correlated with sex hormone-binding globulin (SHBG) secretion in obesity [9].