Testosterone is the main hormone responsible for the development of secondary sex characteristics in men. It is produced mainly in the testicles under the control of pituitary LH and partially by the adrenal glands. About half of blood testosterone is present in an inactive form bound to SHBG, and the other half is in a free form or weakly bound to albumin.The total free and albumin-bound forms constitute bioavailable testosterone. Testosterone levels are higher early in the morning and lower in the late afternoon. Total testosterone levels generally are an accurate representation of the biological activity of natural testosterone or testosterone medication in the tissues. Some conditions, primarily SHBG levels, can affect this interpretation. Levels of SHBG and/or free or bioavailable testosterone can then be used.
Low total testosterone levels can be the result of testicular impairment (high LH) or pituitary impairment (low LH). Other conditions such as alcoholism, certain chronic diseases, anabolic steroids and medications (particularly those used in prostate cancer) are other causes of low testosterone levels. High testosterone levels are most often the result of testosterone supplements, or testicular or adrenal tumours.