In His Miraculous Divine Book, the Holy Quran, Allah, All Mighty, says, 

“Surely! Those who disbelieved in Our Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.): We shall burn them in the Fire. As often as their skins are roasted through, We shall change them for other skins that they may taste the punishment. Truly, Allah is Ever Most Powerful and All Wise.”

[An-Nisa’; 56]

 Consider this part of the above Noble Ayah, 

“As often as their skins are roasted through, We shall change them for other skins that they may taste the punishment”

 This Noble Ayah seems to refer to the place where pain receptors exist, particularly those responsible for burning sensation. In order to taste more of pain: 

“As often as their skins are roasted through, We shall change them for other skins that they may taste the punishment.” 

 Scientists say that the skin is made up of two layers: the epidermis, or outer skin, whose thickness varies according to different body region and according to friction and environmental factors. This layer, the epidermis, consists of three layers: upper, middle and lower. The second layer of the skin is the dermis, which is really incredible, consists of a certain kind of tissue that has capillaries, nerve endings, fat glands, sweat glands, and subcutaneous adipose tissue beneath it. Actually, the dermis  is the most active layer of the skin. 
Now, let's talk about the thyroid gland: hot weather can cause  hyperthyroidism; thyroid gland becomes overactive and produces more thyroid hormones, which  produces heat in order to maintain body temperature. So this is how your body adapt to hot weather.  
 In other words, in hot weather, the thyroid gland produces special hormones which affect the function of other  cells and  activate them. Activation of the cells increases the heat that these cells radiate; and in this way the body adjusts itself to the hot weather. 
 It is really something incredible! If the temperature is 40° C, what happens to our body? Scientists say that there are 200,000 tiny bodies spreading all over the skin. If the temperature rises excessively, these tiny bodies send special nervous messages, which nerve fibers carry to the brain; and the brain, in turn, gives orders to widen all tiny blood vessels all over the body, which causes flushed skin, i.e. that is why our complexion reddens when it is hot.