The disclosure describes techniques for quantifying the autonomic nervous system (ANS) health of a patient with thoracic impedance measurements. Thoracic impedance may be measured utilizing cardiac electrodes and an implantable medical device housing or other electrodes located on or within the patient. Since greater variability in thoracic impedance may indicate better health of the ANS, monitoring impedance changes in a patient may be used to quantify autonomic tone of the ANS, and ultimately, overall patient health. In some examples, thoracic impedance may be measured in response to a change in patient posture for acute monitoring or at predetermined times over several days, weeks, or months for more chronic monitoring of the patient. An implantable medical device may independently analyze the impedance measurements and transmit an alert to the patient or clinician when impedance changes indicate a change in patient health.