Anabolic/androgenic steroids may produce atrophy (shrinkage) of the testicles. Testosterone is synthesized and secreted by the Leydig cells in the testes. Its release is regulated by the hypothalamic-pituitary-testicular axis, a system that is very sensitive to sex steroids. When anabolic steroids are administered, the HPTA will recognize the elevated hormone levels, and respond by reducing the synthesis of testosterone. If the testes are not given ample stimulation, over time they will atrophy, a process that can involve both a loss of testicular volume and shape. This atrophy may or may not be obvious to the individual. In some cases, the testes will appear normal even though their functioning is insufficient. In other cases, shrinkage is very apparent. Visible testicular atrophy is one of the most common side effects of steroid abuse, appearing in more than 50% of all anabolic/androgenic steroid abusers.
Although testicular atrophy is very common in frequency, it is also regarded as a temporary reversible side effect. The gonads, by their nature, will vary in size under hormonal influence. Atrophy should not produce permanent damage. Note, however, that it can be a somewhat persistent issue. It may take many weeks or months of sufficient LH stimulation after steroid discontinuance for original testicular volume to be restored. Likewise, testicular atrophy is usually the root cause of prolonged post-cycle hypogonadism. In extreme cases, full recovery can take more than 12 months, and may even require medical intervention. A post-cycle recovery program inclusive of HCG (which mimics luteinizing hormone activity) may be used to minimize this recovery phase. This drug is also frequently effective for maintaining testicular mass when used on a periodic basis during steroid administration.304 HCG must be used with caution, however, as overuse may cause desensitization of the testes to LH,305 complicating HPTA recovery.
Some of the more potent anabolic/androgenic steroids, including testosterone, nandrolone, trenbolone, and oxymetholone, appear to be more suppressive of testosterone release than many other AAS drugs. This may be explained in part by the additional estrogenic or progestational activity inherent in these steroids, as estrogens and progestins both also provide negative feedback inhibition of testosterone release. It is important to note, however, that all anabolic/androgenic steroids are capable of suppressing testosterone secretion. This includes primarily anabolic compounds such as methenolone and oxandrolone, which are normally regarded as milder in this regard. While these compounds may be less inhibitive of testosterone synthesis under some therapeutic conditions, when taken in the supratherapeutic doses necessary for physique- or performance-enhancement, significant atrophy and suppression are common, and distinctions less pronounced.
Wlliam Llewellyn (2011) - Anabolics