The thermostat is designed to hold the coolant flow until the engine reaches a temperature related to temperature controlled switches and relays. When it opens it still restricts flow to allow the coolant to cool in the radiator prior to circulating back into the engine. If the thermostat is removed you might be okay until you hit a traffic jamb or heavy stop and go traffic.Then it will over heat. Basically removing the thermostat is an invitation to a blown head gasket. When you fill a cooling system as it is a closed system it must be permitted to purge, meaning it will have air in the system that need to pass and then top it off until it is full without air in the system. If it is loosing coolant and not dripping out anywhere that you can see then check it is not blowing white smoke out of the tail pipe. This would be one sign of a blown head gasket. If no smoke and no coolant loss then have a chemical block test done. this is a simple test where coolant is pulled from the radiator with a suction devise filled with a chemical that will turn if exhaust fumes are in the cooling system. if so then it is a blown head gasket.