i The diagnosis of thyroid insufficiency, and the determination of replacement dosing, must be based upon the patient’s symptoms first, and on the
free T4 and
free T3 levels second. The TSH
test helps only to determine the cause. Even here, “normal” may not be good enough. The labs’ reference ranges for free T4 and free T3 are not optimal ranges; but only 95%-inclusive statistical population ranges. The lower limits are below those seen in studies of healthy adults. They define only 2.