My question is related to heating

My question is related to heating. The vehicle will not heat up when idling, but blows warm/hot air air when it is being driven. It appears normal after this, and the temp gauge indicates the appropriate temp. Range. On occassion, the gauge did rise higher, but then returned to normal. I added coolant to the reservoir, but problem continues. I was thinking it may be the thermostat. Any assistance appreciated.