Testosterone is a male steroid sex hormone (androgen) which is produced in the testes and in smaller amounts in the adrenals. Its production is stimulated and controlled by luteinizing hormone (LH), which is manufactured in the pituitary gland. Testosterone works within a negative feedback mechanism: as testosterone increases, LH decreases, whereas increased LH causes decreased testosterone. Levels increase after exercise, but decrease with age. About two-thirds of testosterone circulates in the blood bound to sex-hormone binding protein and slightly less than one-third bound to albumin. A small percent (about 1-4%) circulates as free testosterone. Free Testosterone is Testosterone that is not bound to either Sex Hormone Binding Globulin (SHBG) or Albumin.