Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
Human testicles are made up primarily of two different cell types. Germ cells are responsible for producing sperm , while Leydig cells produce the male sex hormone testosterone. Men in good health typically produce both cell types in roughly equal amounts, and as a result their testes tend to be full, round, and firm. During atrophy, one or both of these cell types tend to die off. This causes a range of internal changes including fluctuation in fluid levels and semen chemistry, but is perhaps most noticeable on the outside: men with this condition tend to have visibly shrunken organs that are soft and loose to the touch.