A Site-Specific Saturable Absorption Model for Transdermal Testosterone Cream

Transdermal testosterone absorption depends strongly on the application site and is not proportional to the applied dose, yet dose planning for compounded testosterone creams is often done as if a fixed percentage were absorbed at every dose. This note describes the cream model used in the TRT Plus app. Weekly absorbed testosterone is computed from the weekly applied dose with a site-specific Michaelis-Menten saturation function, so the absorbed fraction falls as the dose rises. The absorbed amount is converted to an injectable-ester equivalent for comparison with injected protocols, and divided equally across applications to drive a one-compartment first-order absorption and elimination model. Superposition of applications is simulated explicitly to estimate steady-state peak-to-trough fluctuation and time to peak. Five application sites are supported: scrotum, shoulders/arms, abdomen, axilla and inner thighs. We give the formal model, the rationale, simulation conventions and limitations. Per-site model constants are calibrated values and are withheld. The model has not been validated against measured serum concentrations in TRT Plus users and is published as a methods description, not as clinical or dosing guidance.