Thermostats are usually pretty good and do not fail, but I would not bet on it.
I would check the radiator cap and replace that first.
For a 1998 and 172k miles, has the coolant system ever been serviced? If not, you are certainly over the 5yr/100k mile mark.
if you squeeze the upper radiator hose, when car is hot and without burning your hand, and rev the motor up to 2000-3000 rpm you should be able to feel a surge in coolant, that's the water pump pumping. If you don't it may mean the water pump is on it's way out... provided the thermostat is working properly also and is fully open. Would be a good idea to check this before you pull anything apart.
the best way to fill the cooling system is disconnect the small coolant hose on the throttle body, this is roughly the highest point on the engine and makes a good air bleed- keep filling the radiator till coolant comes out the line then reconnect it to the throttle body, fill the radiator to the top and put on the cap. Make sure you're overflow bottle is filled about halfway between the cold and hot mark. Then just run the engine normally, with radiator cap on so it builds pressure, and what little air that's in the system will work it's way out and the system will refill itself from the little extra coolant that's in the overflow bottle. If you fill the radiator slowly and allow the air to come out the throttle body hose, it's small so it takes time, it's a one-step procedure and pretty easy and when you take off the radiator cap the following day to check the level you will find you won't have to add anything.