Are you hot or not?

Photo: Catherine Ledner/Getty Images

A question I ask patients oh, 10 or 15 times a day is, “Have you had a fever?” This question, seemingly so simple, is actually fairly complicated.

Normal body temperature, we’ve all been taught, is 98.6º. This is a bit of medical mythology. Nobody’s temperature is 98.6º all the time. Core body temperature is regulated by an area in the brain called the hypothalamus and it changes throughout the day. We generate heat through various metabolic processes in the liver and muscle (like shivering when we’re cold) and we lose heat through our skin and lungs. Our environment obviously plays a part as well.

“Normal temperature” can vary from 96.0º to 100.8º. Most of us are in the lower end of our range in the morning and reach our peak around 4 in the afternoon. The normal range for women also varies during the menstrual cycle. Temperature often rises after we eat. As we get older, our baseline temperature goes down a little and our ability to mount a fever in response to illness can become less predictable.  This is why your grandmother wears a sweater to the pool in August, and unfortunately, why she can sometimes be a lot sicker than she looks. 

When we get sick, the set-point of our thermostat (the hypothalamus) gets moved up, just like the thermostat in your home, and a message is dispatched to the blood vessel control center in the brain suggesting that you “clamp up”. This vasoactive center issues forth a plethora of chemical messages which cause the blood vessels in your hands and feet to constrict which shunts blood from the skin to the internal organs.  This causes your core body temperature to go up, while you simultaneously feel colder.

People often get hung up on the absolute numbers – I’ll often hear, “My temp is 99.6º but I usually run low, so that’s a real fever for me.”  I’m not so worried about that.  In general, a temperature of 100.4º or more is concerning but I’m more interested in trends. What about today, yesterday, last week, last month? Are you a lot higher in the morning or evening? Does your temp spike and trough during the day? Does this happen a few times a week? Has it been going on for months? What else is going on?

In other words, I want to know exactly when and how hot you really are.

Victoria Rentel MD

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