Taking Your Basal Body Temperature
Your body's temperature when you're completely at rest is your basal body temperature (BBT). The BBT is perhaps the most accurate way to ascertain how well the thyroid gland is working – yes, even more than lab tests! This is because the active form of thyroid hormone (T3) increases our metabolic rate, and our metabolic rate in large part determines our temperature.
The BBT is taken first thing in the morning, before you get out of bed. So put a thermometer next to your bed the night before so it's within easy reach. If it's a non-digital thermometer, before each use make sure it's shaken down well below 96°F. In general, it's best to take your axillary (armpit) temperature, rather than your oral temp. This is because various common conditions can affect the oral temp, such as chronic sinus infections and dental infections that may as yet be undetected. Put the tip of the thermometer high up in your armpit and make sure it's well covered by your arm. Leave it there for 10 minutes, and then read the temperature and record it, along with the date and time. A normal axillary BBT is 97.8 – 98.2°F.
You can also take your temperature in this way in the mid-afternoon, which is when the body's temp is usually the highest. It can sometimes be useful to compare this temp with the BBT when assessing how well the thyroid gland is functioning.
If I have asked you to take your BBT in order to help assess how your thyroid gland is functioning, please take your temp three different mornings and three different afternoons. Record the temps, times and dates and then use the Contact page of this website to send the data to me.
What kind of thermometer is best to use
If possible, use an old-fashioned mercury thermometer.* These are still the most accurate thermometers we have for general use, though they are not sold in drugstores anymore because of the toxicity of mercury. If you don't have one of these, you can purchase the next best thing, which is a Gerathem Mercury-Free Oral thermometer. These can be purchased at most drugstores. The least desirable option would be a digital thermometer because these can vary quite a bit in their read-outs. Remember to wait for the beep if you're using a digital thermometer.
Common things that can interfere with an accurate
body temperature measurement
In women, ovulation can cause a spike in the BBT. Please take your temps during your period if you're a woman who hasn't yet gone through menopause.
Fevers will raise the body temperature, so if you have any kind of infection, if would be best to wait until you're infection-free before taking your BBT.
Exercise, hot baths and showers, and drinking warm/hot liquids can also transiently elevate the temp. Wait at least an hour after exercising before taking your temp.
*Mercury is highly toxic. If a mercury thermometer should break, please follow the guidelines on this EPA webpage: https://www.epa.gov/mercury/what-do-if-mercury-thermometer-breaks