Is it the head gasket

Loosing coolant on daily basis but no signs of a leak. Did have a loose heater hose which eventually came off completely and emptied coolant. However hose repaired and coolant replaced plus pressure tested and no leaks found (tested cold). Still loosing coolant and also heater now only works every now and then. No contamination to oil. Only mild concentrate of antifreeze present recently but water seems to be pumping through rad. I was thinking possible head gasket leaking at rear of engine to cause the leak and frozen bottom end of radiator to cause heater problems. Also thermostat may not be working correctly as temperature remained very low on motorway travel and stayed at 12 o'clock for town driving. However since the cold spell stays at 12 o'clock so possible input of stronger antifreeze solution loosened up the thermostat especially as colour of coolant turned reddish. Any help appreciated.