Hi Melissa,
I don't know the answer to that question, but I do know that you can have MS and have clear MRIs.
You might want to check the health pages, there is something on there about MS mimics, you could see if any of them have the heat intolerance as a symptom.
I'm sorry you are having problems getting a dx. MS is really a hard disease to dx, or at least that's what the neuro's tell us.
I am also undx, and had a clear brain MRI, still have many more tests to go before I can either rule out or rule in MS. I think it's MS, but I'm no dr, so I will have to wait and see what my neuro has to say.
The heat is very hard on me too. Seems like everything goes haywire if I get too warm.
Please take care, and stay with us. This is a great place for support when trying to get a dx, and the info here is second to none.
doni