Testosterone has been subject to abuse, typically at doses higher than recommended for approved indication and in combination with other anabolic androgenic steroids; anabolic androgenic steroid abuse can lead to serious cardiovascular and psychiatric adverse reactions; if testosterone abuse suspected, check serum testosterone concentrations to ensure they are within therapeutic range; consider possibility of testosterone and anabolic androgenic steroid abuse in suspected patients who present with serious cardiovascular or psychiatric adverse events
Testosterone itself possesses a moderate level of Estrogenic activity, whereby it holds a moderate affinity to bind to the aromatase enzyme (the enzyme responsible for the conversion of Testosterone into Estrogen). Therefore, a moderate level of aromatization is to be expected with Testosterone use unless an aromatase inhibitor is utilized (such as Arimidex, Aromasin, and Letrozole) to inhibit the aromatase enzyme and therefore render it unable to aromatize any Testosterone into Estrogen. Therefore, Testosterone serves as a preferable compound for bulking but can also be utilized for cutting and fat loss phases as well. At the end of the day, however, Testosterone of any type is required in any cycle of any anabolic steroid if at least for the maintenance of proper physiological function of Testosterone in the body during a period in which the endogenous production of Testosterone has been suppressed or shut down from the use of anabolic steroids. Testosterone itself is also a very strong anabolic compound, suitable even on its own for any purpose, and is in fact suggested as the very first and only anabolic steroid that should be utilized in all first-time and beginner cycles.
Testosterone is transported in blood bound to a carrier protein, called either testosterone binding globulin (TeBG) or, more commonly, sex hormone binding globulin (SHBG). In healthy men, only about 2% of the circulating testosterone is unbound, with 44% bound to SHBG and 54% bound to serum albumin. The protein-bound testosterone is protected from metabolism in the liver and provides an easily accessible pool of hormone, as the testosterone readily dissociates from its binding protein. Levels of SHBG in plasma are regulated by androgens, oestrogens and thyroid hormones. In healthy men, SHBG levels are fairly constant, but may need to be considered when steroid replacement therapy is used. Plasma testosterone concentrations are around 9–41 nmol/L in healthy men and 1–3 nmol/L in women. Plasma testosterone tends to decrease with age in men. Testosterone is metabolized in the liver, mostly to form androsterone and aetiocholanolone, which are excreted in urine.