As I get older, I'm now approaching 56, I find that I am not acclimating to warmer weather as quickly, and this year almost not at all.  Today, at this time,  it is 75 outside with 31% relative humidity and inside my house is at 56.  I need to get more yard work done, but I'm breaking out into a significant sweat an am quite uncomfortable when outside---to the point where I'm feeling a bit of heat exhaustion.  My exertion level is quite low trying to get the yard work done, compared to shoveling snow in the winter (where I'm quite comfortable).   While inside, cooling off, I'm still sweating and not really feeling cool, but at the warmer end of comfortable.

I'm not overweight by any standard measure, for I stand at 5'7" and weigh about 143 (+/- 3) pounds.

When I go for my physical (once I get employer provided insurance again), what should the doc be looking for:  thyroid issues, metabolic issues, other stuff?  Or, do I need to move to Northern Alaska?

Are there things I can do in the mean time to help?  I drink plenty of water, and will add Gatorade to replenish salt and potassium, along with eating regular potato chips.

Heat related damage/issues can take a cumulative toll on your body. It could really be any number of issues, but I would be careful with the outside to inside air cond. spaces transitions.

I was inactive for a year or two due to back injury problems. It took quite awhile to get my endurance and tolerance up to par. Some folks just dont deal well with the heat as they get older. Just as a note, I am only 3 years behind you.

It's possible you're just getting older.

Also, 75 degrees with 31% humidity is a nice day.   Yesterday, the feels like temperature was 122 here.

Is about 20 degrees really that much of a difference that I'd have to careful with?  It is like going from 90 outside to 75 inside, which in most parts of the country is normal during the summer.

Oh, I don't have air conditioning, but rely on cooling the house down at night by opening windows.

It was 68 degrees inside the other day after work, and I was a bit uncomfortable, though I suffered with 85 degree temps at work that day.

I agree, that 75 should be a nice day to get stuff done in the yard.  But, for me it is no longer the case.

You are 56?  Get a heart workup.

I was thinking the same thing.  It didn't get down to 75 last night here.  

OP you really need to see a doctor.

I've had that done in conjunction with blood pressure issues a couple of years ago, but nothing out of the ordinary there.  Though my heat tolerance was somewhat better, it was still deteriorating at that time.  

Also, how would heart issues cause this?  My heart is not trying to pump hard with the little exertion I've been doing, and in the winter it is not a real problem to shovel 100 yards of driveway, where my heart does pump a bit hard.

I'm 34 and the heat is harder on me than when I was 25. You may not be use to it like you use to be. I wouldn't worry much about it. Just take it easy and drink plenty of fluids when outside. Especially with electrolytes in it. If you start feeling like its too much then take a rest. Heat is no joke.

Switch to barbecue potato chips. 

Is your thermometer accurate? 56* indoor air temp or thermostat setting? 

How much Alcohol do you drink OP?  I know that 'back in my drinkin' days I sweat a lot more and didn't handle heat very well.

75 is too cold to swim

The indoor air temp is read by fairly accurate thermometers.  I don't have a way to control cooling other than cross ventilation at night so it is not thermostat settings.  

My alcohol consumption usually consists of a six pack of beer per week.  

Swimming in 45 degree water right now, I think, would feel quite good.