Like other steroid hormones, testosterone is derived from cholesterol (Figure 1). 5α-DHT binds to the same androgen receptor even more strongly than testosterone, so that its androgenic potency is about 5 times that of T. Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α-dihydrotestosterone (5α-DHT) by the cytoplasmic enzyme 5α-reductase. Androgens such as testosterone have also been found to bind to and activate membrane androgen receptors. The relationship between sex steroids and SHBG in physiological and pathological conditions is complex, as various factors may influence the levels of plasma SHBG, affecting bioavailability of testosterone. Only the free amount of testosterone can bind to an androgenic receptor, which means it has biological activity. Specific proteins include sex hormone-binding globulin (SHBG), which binds testosterone, dihydrotestosterone, estradiol, and other sex steroids.
In conclusion, TRT in adolescent males is hindered by lack of long-term safety and efficacy data and limited options approved for use in this population. Only  TE and  T pellets are US Food and Drug Administration approved for use in adolescent males in the United States. The results indicate that short-term use of  T enanthate (TE) or oral  T undecanoate is safe and effective in inducing puberty and increasing growth in males with CDGP.
Mumps orchitis, a traditionally low-incidence cause of primary hypogonadism in developed countries, appears to be increasing in the United States, with reported cases exceeding 6000, both for 2017 and 2018 . Current estimates suggest that the rate of hypogonadism in pediatric cancer survivors is between 11% and 56% 41-43. This prevalence translates to approximately 1275 affected boys, age 12 to 17 years, in the United States, based on government estimates (US government estimates for 2017, and ).
More specifically, the literature suggests that testosterone may modulate dopamine related activity in (1) the ventral striatum which affects valuation related activation, or (2) modulate the dorsal striatum and bias action selection. In sum, testosterone’s distinct effects on ventral and dorsal striatal regions may represent two pathways for influencing decision-making processes in early adolescence. Furthermore, we recently found a similar positive relationship for pubertal testosterone and impatience (Laube et al., 2017), an important component of impulsive behavior (Romer, 2010). In the current study we sought to elucidate possible mechanisms underlying impulsive behavior in adolescence that are related to the increase in pubertal testosterone.
Very high levels of testosterone (particularly those above the normal range) through supplementation can cause the heart muscle to grow, a condition called cardiac hypertrophy. This means that the effects of TRT use in young men are still uncertain and until longer-term safety studies are performed, many dangers may not yet be known. The side-effects and potential long-term health risks for older men can be quite different from those faced by younger men.
Historically, testosterone therapy was thought to increase the risk of prostate cancer and heart attack. These changes appear to have contributed to lower testosterone even in otherwise healthy young men. So low testosterone is not a natural consequence of ageing that all men will face. In healthy men, the age-related decline probably doesn’t have a negative effect until men are in their 70s and 80s. For more tips from our Medical co-author, including how to reduce stress levels which can limit testosterone production, read on!