A Unified Endocrine–Immune Model Explaining Male Phenotypic Variability: From Androgen Resistance to Immune-Mediated Hair Loss

Abstract

Male phenotypic variability in skin tone, facial hair density, and secondary sexual characteristics reflects complex interactions between genetic determinants of androgen signaling, estrogen metabolism, and immune regulation. Accumulating evidence indicates that androgenetic alopecia (AGA), gynecomastia, and immune responsiveness are not solely determined by circulating testosterone levels, but rather by androgen receptor (AR) sensitivity, local dihydrotestosterone (DHT) activity, aromatase expression, sex hormone–binding globulin (SHBG) levels, and immunogenetic background. Several studies demonstrate that reduced AR sensitivity and altered 5α-reductase activity are associated with attenuated classical androgen-dependent traits, including facial hair growth, while simultaneously modifying susceptibility to hair loss patterns that may clinically mimic androgenetic alopecia. In such cases, hair loss may represent non-androgenic entities, including alopecia areata incognita, chronic telogen effluvium, or post-viral immune-mediated alopecia, particularly following SARS-CoV-2 infection. Gynecomastia represents another clinical manifestation of altered androgen–estrogen balance. Increased aromatization of testosterone, heightened estrogen receptor (ER-α/ER-β) sensitivity, or diminished androgen-mediated counter-regulation can shift the estrogen-to-androgen signaling ratio toward estrogen dominance, even in the presence of normal serum testosterone levels. Genetic variants affecting CYP19A1 (aromatase), AR polymorphisms, and SHBG concentrations have been implicated in this imbalance. Sex hormones exert profound immunomodulatory effects. Estrogens enhance humoral immunity, promote Th2-skewed immune responses, and lower the threshold for autoimmune activation, whereas androgens generally exert immunosuppressive effects. This hormonal divergence partly explains the higher prevalence of autoimmune diseases in females and suggests that males with reduced androgen signaling and relative estrogen predominance may exhibit an immune phenotype intermediate between typical male and female patterns. Taken together, this narrative review proposes a unified endocrine–immune phenotype characterized by reduced androgen receptor sensitivity and relative estrogen dominance in a subset of men.