The happy homeotherm 24. 十一月 2010 William Davis (14) If you were a "cold blooded" poikilotherm unable to regulate internal body temperature, you would have to sun yourself on rocks to raise your body temperature, just like turtles and snakes. When it got cold, your metabolic rate would slow and you might burrow into the mud to hide. You and I, however, are homeotherms, terrestrial animals able to regulate our own internal body temperature. Principal responsibility for keeping your body temperature regulated falls with the thyroid gland, your very own thermoregulatory "thermostat." But internal body temperature, even in a homeotherm, varies with circadian rhythm: Highest temperature occurs in the early evening around 8 p.m.; the low temperature nadir occurs at around 4 a.m. The notion that normal human temperature is 98.6 degrees Fahrenheit is a widely-held fiction, a legacy of the extraordinary experience of 19th century German physician, Carl Reinhold August Wunderlich, who claims to have measured temperatures of one million people using his crude, uncalibrated thermometer to obtain axillary (armpit) body temperatures. Dr. Broda Barnes was a 20th century American proponent of using the nadir body temperature to gauge thyroid function. Like Wunderlich, Barnes also used axillary temperatures. Modern temperature assessments have employed radiotransmitting thermistors that are swallowed, with temperatures tracked as the thermistor travels through the stomach, duodenum, small intestine, large intestine, rectum, then peek-a-boos back out. Such internal "core temperature" assessments have shown that:--Axillary temperatures do not track with internal core temperatures very well, often veering off course due to external factors.--Axillary temperatures are subject to ambient temperatures, such as room temperature, and are affected by clothing. --Axillary temperatures are more susceptible to physical activity, e.g., increased with exercise or physical work. Even right vs. left axillary temperatures have been shown to vary up to 2 degrees Fahrenheit. Studies such as this demonstrate that normal oral temperature upon arising is around 97.2-97.3 degrees Fahrenheit. While we lack data correlating thyroid function with circadian temperature variation, the a.m. nadir does indeed, as Dr. Barnes originally suggested, seem to track thyroid status quite well: lower with hypothyroidism, higher with normal or hyperthyroidism. I have been using 97.3 degrees F orally as the cutoff for confirming or uncovering thyroid dysfunction, particularly when symptoms or blood tests (TSH, free T3, free T4) are equivocal, a value that has held up well in the majority of cases. I find it helpful when, for instance, someone complains of cold hands and feet and has normal TSH (1.5 mIU/L or less in my view) but low free T3. An a.m. oral temperature of, say, 95.7 degrees F, suggests that there will be a favorable response to T3 supplementation. And it nearly always plays out that way. Wouldn't it be interesting to know if there was insight into thyroid status provided by also examining the circadian behavior of temperature (e.g., height or timing of the peak)?