Fig. 5: The interaction between testosterone metabolism and the intestinal microbiome (testobolome).

Testosterone, whether endogenous (blue box) or exogenously acquired (yellow box), circulates systemically in the bloodstream (green arrows) and is gradually inactivated by the liver and, to a lesser extent, the kidneys, where it undergoes sequential modification: the addition or exposure of hydroxyl (–OH) or keto ( = O) groups to increase polarity (phase I metabolism), followed by conjugation with charged groups like glucuronic acid or sulfate (phase II metabolism) to increase water solubility¹⁸⁰. The liver is considered the central site for testosterone metabolism and bioactivity regulation, as it not only catalyzes hormone clearance but also produces albumin and SHBG, which bind circulating testosterone and govern its systemic availability. Conjugation renders previously hydrophobic sex steroid hormone hydrophilic, facilitating the excretion of testosterone metabolites primarily via urine (after renal or hepatic processing) or secondarily via bile (hepatic processing), followed by fecal elimination. In the lumen of the gastrointestinal (GI) tract, conjugated hormones secreted in bile may undergo microbial transformation via the testobolome (pink dashed box), including deconjugation and structural modifications (purple box). These microbial processes can reactivate bioactive sex steroids otherwise destined for excretion. The resulting compounds may then be reabsorbed into the bloodstream, contributing to enterohepatic recirculation (red arrows) and potentially elevating systemic sex steroid levels. Among the organs involved, only the liver and intestines contribute meaningfully to testosterone reabsorption, while the kidneys primarily serve as an excretory route. SHBG – sex hormone-binding globulin, CBG – corticosteroid-binding globulin. Figure created with BioRender.