Fig. 1. Potential mechanisms linking obstructive sleep apnea (OSA) and obesity to low testosterone. A bidirectional relationship between testosterone and obesity or OSA is shown. While OSA and obesity lead to lower testosterone, low testosterone promotes obesity and affects sleep quality. REM: rapid eye movement, SHBG: sex hormone binding globulin, GnRH: Gonadotropin-releasing hormone, LH: luteinizing hormone.