BASAL METABOLIC RATE S INCE 1910 when Benedict introduced a new apparatus for determining the respiratory exchange, the basal metabolic rate has been established as a routine clinical aid in the diagnosis of thyroid disease. As is true of any laboratory test, the determination of the basal metabolic rate has definite limitations and requires proper interpretation.Among the errors of interpretation to be avoided is that of accepting an elevated metabolic rate as certain evidence of the presence of hyperthyroidism or, similarly, interpreting a moderately low rate as an indication of thyroid insufficiency. The tendency of some to disregard the basal metabolic rate as an aid to thyroid diagnosis is equally unjustified. Clinicians who have an extended experience with thyroid disease might well dispense with the basal metabolic test and depend entirely on clinical judgment, but even then occasions would arise when they would desire to use the test.There are certain specific variants to be understood and accepted in the interpretation of any basal metabolic rate in relation to thyroid disease besides the accepted fact that the normal range of the rate is fairly wide.(a) The metabolic test may not be basal. (b) An elevated basal metabolic rate may occur in the absence of hyperthyroidism.(c) Hyperthyroidism, even severe, may occur in a small number of exceptional cases in the presence of a normal basal metabolic rate.(d) A low basal metabolic rate may occur in the absence of clinical myxedema.(e) Myxedema, even profound, may occur in the presence of only a slightly lowered basal metabolic rate.