My 2002 Altima is overheating. The radiator hose is hot. I am losing antifreeze from the overflow resorvoir. Do I need to replace the thermostat?

Couple of thoughts. are you losing water/antifreeze, do you have the right mixture 50/50 water/antifreeze. You did not say if the thermostat is removed or replaced. If thermostat is stuck closed water will no circulate. Are you getting any flow from the water pump. If your heater core is bypassed you should not get any hot air from the heater in the vehicle. Make sure your oil levels are good.
https://www.dummies.com/home-garden/car-repair/heating-cooling-system/how-to-troubleshoot-an-overheating-engine/
https://www.thoughtco.com/figure-out-why-your-car-is-overheating-281320

Well, I would say that the radiator would be next on your list, but usually the overflow is attached to the radiator, so that should be impossible to boil before radiator. How does the temperature gauge look? Even if any parts are hot to the touch that doesn't necessarily mean they are overheated. It may be that the system is still working out bubbles, hence the overflow boiling over.

John sounds like your sensor may be bad. Next it could be your thermostat sticking use a temp gun and measure the temp of your radiator at the cap. Do not remove cap. Just take temp. If it is getting hot it should spit water out. Check to see if the electric fan is coming on the sensor to cut the fan on maybe bad or the fan. Hope this helps.

It would appear your thermostat is stuck in the closed position , find where it is on your engine, drain coolant and remove thermostat housing and stat, replace housing(liquid gasket OK) and coolant. now run engine, turn heater controls to hot leaving filler cap off, top up any water to max and replace cap now warm up to temp until fan comes on. if all ok replace thermostat with a new one and new gasket, you could also flush the sytem out at the same time with a hose pipe by loosening one of coolent pipes and flush radiator.

engine running too hot.check coolant level, should be 50/50 antifreeze and water.replace thermostat and radiator pressure cap.check antifreeze level in the radiator,coolant level should be just below radiator cap, add coolant to overflow jug,should alway be at the full cold mark.empty coolant overflow jug will allow air be drawed in the coolant system causing overheating.if overheating happens when coming to a long stop.could have faulty engine coolant temperature sensor or bad coolant fan fuse or relay.

Then replace the thermostat, because if on lower hose not hot and on upper hose scalding hot = antifreeze flow blocked by the thermostat wich remains closed!!!

Thermostat replacing:

Overheating can be numerous things: Fans not working, water pump, thermostat, overflow cap, and the list keeps going. When the engine is running and heats ups, feel the heat of the top hose, if the hose isn't hot, the thermostat is most likely stuck open and needs replaced. If it is hot we know you have circulation. The cap on the expansion tank may be defective, if it can't hold the pressure of the water, it will release at the overflow cap if the cap is bad. Also check the coolant level.

the water is getting to hot even though its not overheating because you need a thermostat to trap the water in the radiator to give the water a chance to cool down then the thermostat opens and the cold wanter enters the motor and the hot water in the motoer enters the rad. to cool down. with no thermostat, this doesnt happen it just circulates and warm up to the temp it warms up to and is at the same temp in motor and rad then to hot will come out the overflow, as a resualt, you will wind up losing antifreeze then wont have enough antifreeze in your engine then will damage your engine.Put a thermostat in and a water pump. fix it right or you will damage your car.

there is a coolant cap on the overflow bottle check it for seal
i'd just replace it
also if you put the new thermostat in backward the overheating would be significant

the best thing to get is the Haynes manual for your car at advanced auto or autozone or somewhere like that and the book will tell you how to take everything apart and put it back. i think the manual is about $16.00