I have pressure build up in the radiator but no leaks found. The water pump was checked an nothing there. Mechanic thinks it could be a head gasket, but everything I have read stated that I would see the anti-freeze in the oil. Nothing in the oil. No oil leak no leak from hoses or radiator. But losing fluid from the plastic bottle and it overheats. Any suggestions of what it could be. I will be buying a new thermostat and starting there.
It could be a head gasket and lack of oil in the coolant or coolant in the oil is NOT a conclusive indicator. It all depends on where the defect is.
But there are a number of other possibilities that you should investigate before checking the head gasket.
Check the radiator cap, check the hoses, check that the radiator is no clogged, check that there are no air bubbles in the system.
After all the simpler problems are ruled out, then you can investigate the head gasket.