Abstract
Intracrinology refers to the local synthesis, activation, and inactivation of sex steroids within peripheral tissues from precursor hormones such as dehydroepiandrosterone (DHEA), androstenedione, and testosterone (T). This process occurs largely independent of circulating hormone levels. With aging, declining adrenal and gonadal androgens create tissue-specific hormone imbalances that contribute to common symptoms in both men and women, including loss of muscle and bone mass, reduced libido, mood disturbances, low energy, and increased body fat. Conventional testosterone replacement therapy (TRT) guidelines focus on restoring serum testosterone to mid- or upper-normal ranges for young men, or to premenopausal levels in women, to avoid pharmacologic dosing. However, these serum-centric approaches fail to account for the critical role of intracrine production. In peripheral tissues, most bioactive testosterone, dihydrotestosterone (DHT) via 5α-reductase, and estradiol (E2) via aromatase are produced locally from adrenal precursors rather than from circulating testosterone alone. Continuous-release subcutaneous testosterone pellet therapy delivers stable physiologic levels of testosterone for 3-6 months. This provides a consistent substrate that allows tissues to produce DHT and E2 on demand, according to local enzyme activity, without the peaks and troughs associated with gels, injections, or oral formulations. The result is an enzyme-aware, symptom-driven model that prioritizes clinical symptom relief, individual enzyme profiles, and side effect patterns over strict serum targets. This narrative review explores the roles of steroidogenic enzymes across organ systems, the tissue-specific effects of testosterone, DHT, and estradiol, strategies for managing conversion imbalances (including adjunctive aromatase and 5α-reductase inhibitors), the limitations of serum estradiol monitoring, and observational evidence supporting higher steady-state testosterone levels for effective intracrine restoration in aging populations. While promising for individualized hormone optimization with acceptable safety in monitored patients, this approach requires validation through large-scale randomized controlled trials.