Every time I use a heat lamp to heat my rodents their bellies bust. I recommend thawing overnight in the fridge (which is the safest way to thaw any meat) then warming in a plastic bag in hot water. If you try to thaw in hot water you will be getting the outside of the rat too warm while the inside is still frozen. I put it in some hot water for maybe 10 minutes, then blast it with a hair dryer until it feels warm to the touch. That works for me. Just make sure you're getting the f/t rats really warm, as their natural body temp is around 100 degrees.