In this paper, we present a new flexure-based load cell with adjustable stiffness designed for force sensors with extended force range and respective measuring resolution. This novel monolithic mechanism consists of a T-shaped flexure pivot combined with an adjustable elastic preloading element. In particular, the compression preload can be adjusted to reach near-zero stiffness, i.e., very high sensor sensitivity, or to reach negative stiffness, i.e., a bistable behavior leading to a passive force-limiting device. The analytical model based on Euler-Bernoulli beam theory is derived in order to extract a non-linear formula of the pivot stiffness as a function of its preload. The results of finite element simulations carried out to verify the analytical model are presented. Based on these models, a mesoscale load cell has been dimensioned aiming for a force resolution of 50 nN when tuned to near-zero positive stiffness, approaching that of micro-force sensing nanoprobes.