Methods for treating fluid overload in a subject comprise shifting fluids directly and non-invasively from an interstitial compartment of the subject to skin of the subject through controlled local sweating. Methods of the invention allow for removal of excess fluid from the interstitial compartment of the subject and treat fluid overload in the subject. Sweat stimulation systems comprise a chamber and first and second relative humidity sensors. The chamber is sized to fit around a body part of a subject, comprises an inlet and an outlet, and is configured such that air flows through the chamber from the inlet to the outlet. The first relative humidity sensor is operably located inside the inlet, and the second relative humidity sensor is operably located proximate the outlet.