One of the more common medical misconceptions is that the average body temperature is 98.6°F. This number is based on research done in the early 1800s, and was passed from generation to generation with few people questioning its validity. The actual average body temperature is about 97.6°. Women have a slightly higher (0.3° F) body temperature than men. The average temperature in elderly people is approximately 96.8° F. It is possible, although speculative, that the average body temperature was indeed 98.6°F in the early 1800s, with an interim reduction in body temperature resulting from a progressive impairment in thyroid function. This is not as far-fetched as it may seem. In that same time period, women have reached menarche (the first menstrual period) at a significantly younger age. The primary determinant of menarche originates in the hypothalamus, and is mediated through the pituitary gland (1). The age at which menarche occurs is influenced by socioeconomic, genetic, nutritional, and general health factors. These factors affect the timing of the hypothalamic signal to the pituitary to begin menstruation. Since the hypothalamus controls the pituitary, and since external factors can influence the hypothalamus, it is not surprisingthat these factors might also affect thyroid activity and, consequently, body temperature.

(1) The hypothalamus is the part of the brain that controls endocrine activity, such as thyroid and adrenal function. The pituitary is the so-called “master gland.” An analogy might be that the pituitary is the thermostat in your home, and you are the hypothalamus. The temperature of your home is regulated by the thermostat, but the thermostat is controlled by you.

Another frequently neglected concept regarding body temperature is that of its diurnal (daily) variation. While the average body temperature is 97.6°, normal body temperature actually varies from a low of about 96.6° in the early morning hours to 98.6° or above when the person is active later in the day. Vigorous exercise can easily raise body temperature into the “fever” range. Smoking, caffeine, stress, consuming hot food, and exposure to a hot environment can also elevate body temperature.

Can you trust ear thermometers? You have probably seen them used in a hospital or doctor’s office, and they are available for in-home use. Because they can determine the temperature in a second, they are convenient to use on fussy children. But, to return to my original question, can you trust them? Given that they are used in hospitals, it would seem logical to assume that they are accurate. While they can be accurate, they can also be very inaccurate, too. When they err, they tend to issue a falsely low reading—precisely what you don’t want when you are trying to gauge whether a fever is present.

When I worked in the ER, it was not unusual to be told that a patient’s temperature was 90° F, even when the patient obviously had a normal temperature (which was then confirmed using a rectal thermometer). While there are several reasons why they can be inaccurate, I’ll spare you from a lecture in physics and simply point out how you can use them so that their readings are reasonably reliable. To achieve the most reliable result:

• The ear canal should not have excessive wax (cerumen) present.
• The speculum of the thermometer should be pointed at the ear drum. Because the ear canal curves somewhat, this is not as easy as it sounds. To straighten the canal, grasp the outer ear (auricle) at the 10 o’clock position and gently pull up, back, and slightly out.
• Take the temperature several times, and consider the highest reading to be the actual temperature.
• Practice using the instrument until you get the knack of it. Since the highest result is generally the most accurate, you can use this fact as feedback to determine when you have mastered the technique. It is best to experiment with the thermometer before you actually need it. It is obviously easier to obtain the cooperation of a person when they are not sick (especially in the case of children), and this will give you a reasonable idea of the person’s baseline body temperature.

Posted by Dr. Pezzi on Sunday, May 18th, 2008 at 8:34 pm.