What causes low body temperature in an adult?

Low body temperature. The vast majority of low temperatures in adults are actually normal. We say 98.6 is the normal temperature, but actually normal is a range from approximately 97 to 99. The rare pathologic and clinically significant causes of low body temperature are hypothyroidism, hypothermia, severe infection in the blood stream, shock, and the period just after a high fever (often your body overcools itself).
Hormone problems. An underactive thyroid is the most common and easily treatable cause. Other hormones can affect temperature as well. Women generally have a higher body temperature after ovulation due to the effects of progesterone. Addison's disease (not enough adrenal hormones), liver disease and diabetes can all lower body temperature. Hypothermia (prolonged exposure to the cold) will lower body temp as well.