dumb question, is the level low? or do you have any leaks? there also may be an air bubble in the system, take the cap off the coolant tank by the turbo, if its low fill it. Start the car and turn the heat on, let it idle,wait and see if the level drops at all, or if it gurgles a little. i usally let it run until the fans turn on, checking the temp guage often.if that checks out ok, then drain the coolant and change the thermostat, I would also check the condition of the hoses, because that would be a good time to change those as well.
Dom Tuned 2003 WRX Wagon
If it's worth doing, it's worth overdoing