Testosterone levels are regulated by the hypothalamus, pituitary gland, and gonads. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on the Leydig cells in the testes to produce testosterone. Testosterone levels are highest in the morning and decline throughout the day.