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Aim: The reason why non-calibrated hair variables poorly estimate scalp hair coverage during hair growth studies was studied. Methods: Hair productivity integrates density, diameter and daily hair growth rate. Cross-sectional studies have established hair productivity in female and male patients (480 controls vs. 90 controls) with self-evaluation of hair loss, phototrichogram (CE-PTG-EC) and scalp coverage scoring, (SCS). Tracking productivity of individual hair follicular units from longitudinal studies challenged the application of our methods during drug trials. Results: Hair loss means decreased productivity and increased “time to complete coverage”. The hair mass index (HMI) linearly connects productivity with clinical perception of coverage, i.e., SCS. The ensuing HMI abacus translates independently of gender, age, pattern or severity and unravelled unequal intervals between categories of the Ludwig and Hamilton classifications. With one severity grade shift, time to complete coverage varied from 21-51 days, i.e., no equality. During longitudinal studies, SCS detected improved productivity, reflecting clinically relevant responses, but remained stable in the absence of significant productivity variations. Follicular unit labelling and individual hair growth tracing showed that reversal of miniaturised hair follicles does not play a major role during drug-induced hair regrowth. The latter reflects re-activation of resting-dormant terminal hair follicles. The recovered productivity would not be possible once hair follicles enter the phase of structural-functional irreversible miniaturisation.Conclusion: Besides pattern identification, density of nanohair and HMI appear as innovative diagnostic approaches. Abrupt transformation (within one cycle) of terminal hair follicles into miniaturised ones and its reversal as the effect of active FDA-approved drugs remain highly improbable.