Your body has a "thermostat" which regulates your body. In order for your body to function properly "all your wiring" has to be in good working order. When your nerves have been affected with lesions and subsequent scar tissue, the "thermostat" no longer works well. Some MSers are not affected by temperatures. For me, in 1982-2002 I had problems if I had heat from my neck up, but after 2002, my "thermostat went on the fritz". Since then, I have discovered I have a high and low temperature range. The upper one varies depending on the humidity, but my low range is about 52 F (11.11 C). Below that I have problems.
There are other diseases which can also make you heat intolerant. If I go out in the heat, I find that either I will go into an uncontrollable sweat or my energy suddenly plummets and I become fatigued. During the summer months, I will "hibernate" during the heat of the day.