PIH recommends that 1st AM basal temperature between 97.8 °F (36.6 °C) and 98.2 °F (36.8 °C) based on the Barnes Basal Temperature Test*
* Broda Barnes MD, developed and promoted a diagnostic test for thyroid function that became known as the “Barnes Basal Temperature Test”. This test is performed by placing a thermometer in the armpit for 10 minutes immediately upon waking. Barnes considered a measurement of 97.8 °F (36.6 °C) or below to be highly indicative of hypothyroidism, especially when hypothyroid symptoms are present. Barnes believed that a reading over 98.2 °F (36.8 °C) was indicative of hyperthyroidism. This can sometimes be superior to thyroid tests and predict thyroid dysfunction more accurately. According to Barnes many common diseases, including heart disease, cancer, depression, arthritis, diabetes, the common cold, tonsillitis, ear infections, apparent laziness in children, various menstrual disorders, and skin disorders, were all caused or exacerbated by hypothyroidism.
– Barnes, Broda Hypothyroidism: the Unsuspected Illness. HarperCollins. (1976) ISBN 0-690-01029-X.