Do Serum Testosterone Values Potentiate or Predict Muscle Hypertrophy? A Structured Narrative Review

Testosterone is the prototypical anabolic-androgenic hormone, and circulating concentrations in men are widely assumed to predict the magnitude of resistance-training (RT)-induced hypertrophy. The available evidence does not support this assumption. This structured narrative review synthesizes the evidence relating testosterone levels to RT-induced hypertrophic responses across the full testosterone spectrum, from exogenously administered dose–response trials to eugonadal training cohorts, androgen-suppression and androgen-deprivation trials, and the female parallel. We propose a three-phase model of the testosterone–hypertrophy relationship: (i) a lower-bound zone of severe testosterone deficiency, in which the hypertrophic response to resistance training is attenuated but not abolished; (ii) a null, non-predictive zone across the male eugonadal range; and (iii) a graded, dose-dependent augmentation of hypertrophy from exogenous testosterone. A single linear model, in which more testosterone always means more muscle growth, does not fit this pattern. In summary, testosterone appears constrained under severe deficiency, permissive within the normal male range, and a graded driver of lean-mass gain when administered exogenously. The transition between severe deficiency and the eugonadal range remains largely untested in resistance-training studies in healthy men.