Pork should always be cooked to an internal temperature of 145 degrees according to the FDA and should rest for at least 3 minutes. This will result in more medium-rare pork. Pork cooked to 150-155 degrees is considering medium. Pork cooked to 155-160 degrees is considered medium well and pork cooked to 165 degrees is considered well done.
Since pork tenderloin is a leaner cut, cooking it to medium or medium well is recommended so that it stays tender. Use a meat thermometer for the most accurate reading.