The Hypothalamic Pituitary Testicular Axis (HPTA) refers to the ability of brain and hypothalamus (master gland) in particular, to interact with hypophysis, the gland located within the sella turcica bone of the sphenoid bone, with gonads and testicles particularly in male individuals. Androgenic Anabolic Steroids are synthetic derivatives of testosterone, the major androgen in males. Testosterone itself suppresses majorly the HPTA, while several other derivatives do so to a greater or lesser extent.
Moore: http://www.anabolic.org/getting-to-know-the-hpta/
Getting to Know the HPTA