The Barnes Basal Temperature Test (BBTT) is a simple test anyone can undertake at home in order to assess their thyroid function. It is most often used to detect undiagnosed hypothyroidism (underactive thyroid) but will also provide evidence of hyperthyroidism (overactive thyroid).
The test gets its name from the doctor who first proposed it - Broda Barnes MD in the 1970s book, Hypothyroidism: The Unsuspected Illness. Dr. Barnes is seen my many in the field of integrative, functional and alternative medicine as the grandaddy of the thyroid field and the man responsible for ending the suffering of many thousands of patients with underactive thyroids who had not been diagnosed because their blood tests appeared normal and they were dismissed by their doctors as healthy despite their persistent symptoms. Most doctors even today only test thyroid-stimulating hormone (TSH) levels and sometimes thyroxine (T4). Normal levels of these however may not confirm healthy thyroid function as thyroxine must be converted to triiodothyronine (T3) which must efficiently bind to cell receptors for the functions of the thyroid to be expressed. Conversion of T4 to T3 is influenced by many factors including levels of selenium and other nutrients in the body and the presence of toxins. Receptor function can also be affected by pollutants and other factors.
Dr. Barnes put his many decades of experience into a succinct and easily-digestible form in his book and explained the proper rationale for the test based on his clinical experience and the medical literature.
The thyroid gland can be seen as the gas/accelerator pedal of a car. It controls the body's rate of metabolism; the speed at which every physiological event in the body takes place. As a consequence of the thyroid's functions it influences body temperature. Dr. Barnes felt that by measuring the basal body temperature (that when the body is inactive) a good indication of the thyroid's functional status could be obtained.
He recommended that you measure your left underarm temperature for ten days in a row first thing in the morning before doing anything else at all (going to the toilet etc). Movement and activity of any kind raises body temperature and invalidates the results. The thermometer should be shaken down at night before going to sleep to avoid having to do so in the morning (unless of course it is a digital thermometer). Menstruating women should start this ten-day period on the third day of their cycle. Many health care practioners who recommend the test now suggest a longer time period but Dr. Barnes felt 10 days was enough for an accurate assessment of thyroid function.
The underarm temperature is actually lower than that measured under the tongue so 36.6-36.8C is considered normal in the BBTT. Anything below 36.6C is considered an indicator of possible underactive thyroid function while anything above 36.8C could indicate an overactive thyroid (although of course it may be a sign of infection and fever or other problems).
Although the BBTT can be a good indicator of thyroid function it is not infallible and should be taken in the context of symptoms, medical history and blood tests.