And the latest theory (guess) is...

hypothyroid.

hypothalamus
Functions:
• Body Temperature
• Emotions
• Hunger
• Thirst
• Circadian Rhythms
The hypothalamus is composed of several different areas and is located at the base of the brain. Although it is the size of only a pea (about 1/300 of the total brain weight), the hypothalamus is responsible for some very important functions. One important function of the hypothalamus is the control of body temperature. The hypothalamus acts as a "thermostat" by sensing changes in body temperature and then sending signals to adjust the temperature. For example, if you are too hot, the hypothalamus detects this and then sends a signal to expand the capillaries in your skin. This causes blood to be cooled faster. The hypothalamus also controls the pituitary.

http://www.mall-net.com/mcs/coldbody.html

http://www.nlm.nih.gov/medlineplus/thyroiddiseases.html