This important and crucial test, though familiar to those who have a special interest in hypothyroidism, is not taught in medical schools in the UK, and hence most practitioners are unfamiliar with it. Sadly, this unfamiliarity causes them to criticise the test simply on these grounds.
The Basal (at rest) Temperature Test was first described by Dr Broda Barnes from the USA in 1942 in The Journal of the American Medical Association, and later in the Lancet, in 1945. He had noted that, when screening patients prior to testing them for the Basal Metabolic Rate (BMR), their temperatures were consistently lower when the results of the test showed the lowered metabolism of hypothyroidism. The BMR test involves the resting patient having their oxygen consumption and heat output being carefully measured over a time interval. It became clear that the relationship between low BMR and low body temperature was so consistent, that the temperature under basal conditions alone was sufficient to make the diagnosis.
It is this test, which has proved to be so valuable in the diagnosis of hypothyroidism. It is not intended that it should be taken solely by itself as diagnostic, but as part of an overall clinical appraisal, where the history of the illness and physical examination are considered as a whole. Where blood tests are apparently normal, it may nevertheless point very clearly to the true diagnosis as there are few causes other than hypothyroidism which produce a low basal temperature. These include malnutrition (or crash dieting), alcoholism, liver failure and of course hypothermia.
Barnes used a mercury thermometer but mercury thermometers are no longer available to purchase under The Medicines and Healthcare products Regulatory Agency (MHRA) Council Directive 2007/51/EC. However, mercury thermometers already in use are not affected by this Directive.
To do a basal temperature test, it should be done as soon as you wake up and before you get out of bed. In women who are menstruating, their body temperature varies with the cycle; creating errors which can be avoided if the basal temperature is taken on days 2, 3, 4 and 5 of the cycle. Men and non-menstruating women may take their temperature on any day.
It doesn't really matter which kind of thermometer you use when doing the basal temperature test, although it is important to be consistent in how you do it:
If using a mercury thermometer it's a good idea to have it already shaken down the night before and placed on your bedside table. To take your temperature you can place it either under your arm for 10 minutes or under your tongue for 3 minutes.
If using a digital thermometer you should place it under your tongue not your arm – a digital thermometer is not to be relied upon if placed under the arm due to the speed of the reading. Place it under your tongue until the thermometer beeps or buzzes, or for the length of time suggested in the instructions.
Note the reading, and do it for several mornings so the results can be averaged out, since they may vary slightly day by day.
If you have taken your temperature under the arm, the normal temperature is:
36.6ºC - 36.8ºC (97.8ºF - 98.2ºF)
If your temperature is below 36.6ºC (97.8ºF) then hypothyroidism should be considered if you have symptoms.
If your temperature is above 37.0ºC (98.4ºF), hyperthyroidism is possible if symptoms are present and if there is no other illness present to cause a fever.
If you have taken your temperature under the tongue, the normal temperature is:
Normal temperature is 36.5ºC - 37.2ºC (97.7ºF – 99.0ºF)
If your temperature is below 36.5ºC (97.7ºF), hypothyroidism should be considered if symptoms are present.
If your temperature is above 37.4ºC (99.2ºF), hyperthyroidism is possible if symptoms are present and if there is no other illness present to cause a fever.
This test is a guide only as some temperature variations could be due to infection, virus, etc. This test, used in conjunction with the Thyroid UK Signs and Symptoms Lists, can be helpful.