An adherent device to monitor a tissue hydration of a patient comprises an adhesive patch to adhere to a skin of the patient. At least four electrodes are connected to the patch and capable of electrically coupling to the patient. Impedance circuitry is coupled to the at least four electrodes to measure a tissue resistance of the patient, where the circuitry is configured to determine the tissue hydration in response to tissue resistance. The circuitry may comprise a processor system and the tissue resistance may correspond to a change in patient body fluid. The impedance circuitry is configured to measure the hydration signal using at least one low measurement frequency, which may be in the range of 0 to 10 kHz. Multiple measurement frequencies may be used and the hydration signal may include a tissue reactance measurement.