I have a Tenderloin that was roasted to medium and seared. How can I best reheat it without it getting dry and tough?
Bring it back to the old school, which is place a covered plate in a 125-200 degree oven until just warmed through. It was the option of choice pre microwave, and still the best way for roasts and pastas IMO. It should come out steaming after about 20 minutes, depending on temp. You lose less moisture the lower you go, but it takes longer so you get to choose the median point between those that's good for you.
It's probably too late for this particular instance, but something to keep in mind for the future. If you make/eat a lot of roasted meats, such as tenderloin, prime rib, pulled pork, etc., you would be wise to invest in a Foodsaver. Divide your leftovers into single-size portions, seal them in vacuum bags, and freeze them. When it's time to reheat, just drop a bag into boiling water and reheat. You will reheat your food without losing any moisture.