Car leaking antifreeze

The thermostat housing attaches to the front of the head and is sealed by a rubber o-ring. My experience is that if the antifreeze leak is at the front of the engine it is either this o-ring needing replaced or one of the small by-pass coolant lines has developed a leak. I also replace the factory hose clamps with new screw type hose clamps as these secure better than the factory clamps. If the leak is on the driver side of the engine the heater core supply and return hose could be at fault.