If i am having temperature sensitivity due to hypothyroidism, am i more susceptible to the extreme summer heat?

Yes: Since your level of thyroid function is most definitely associated with how you feel the ambient temperature, i would say most definitely. What is odd in my experience is that people do not always behave exactly as the books say they should. A person with very low thyroid function may prefer it very hot, but usually any thyroid disorder is associated with not handling hot or cold very well.

Not if managed: If your thyroid hormone levels are normal, you should be no more susceptible to the heat than the average person.