In this article, we present a theoretical analysis of a junctionless (JL), ion-sensitive, field-effect-transistor (ISFET), self-consistently combining the electrochemical interaction between the semiconductor-insulator interface and the surrounding electrolyte medium. Incorporating charge-based core relationships for the nanowire (NW) and planar double gate (DG) JL FETs with basic relations governing the electrolyte-insulator proton exchanges, we predict the output characteristics of the NW and DG JL ISFETs with respect to pH for all the regions of operation. This hybrid charge-based approach of JL ISFETs is fully validated by COMSOL Multiphysics simulations without the need to introduce any fitting parameters. These developments are suitable for implementation in circuit simulators as well as for fast prototyping by tuning the technological parameters and estimate their impact on the device performances, including the electrolyte medium.