Hi dear user I have gone through your question. I can understand your concern. Heat exhaustion is a heat-related illness that can occur after you ve been exposed to high temperatures, and it often is accompanied by dehydration.Without proper intervention, heat exhaustion can progress to heat stroke, which can damage the brain and other vital organs, and even cause death. Your symptoms like feeling extremely tired can be attributed to your actual hypothyroid state and heat axhaustion in your state can be dangerous because dehidration can result in electrolite imbalance,hypovolemic shock,Kidney failure,Seizures,cerebral edemaend even coma. Hope I have answered your question. If you have any further questions I will be happy to help. Wish you good health.