Ask the Doctor Free T3
"I understand the measurement of free t3 is the most accurate evaluation of thyroid function. What methods are now available to measure the free t3 concentration? How do I get my doctor to order the test?" -- SK, Los Altos, CA
Free t3 levels are not the most accurate indicator of thyroid function, and have limited applicability. Free t3 measurements. Like free t4, measure the hormone that is not bound to binding proteins such as TBG (see Levothyroxine Dosage and Estrogens). Free hormone measurements are difficult since concentrations measured are very small - in fact the measurements actually estimate the free levels rather than measuring them directly.
I have found that newer free t3 test results have been quite variable and somewhat unreliable. Serum TSH measurements remain the most sensitive indicator of thyroid status (in the absence of pituitary disease).
Sometimes the serum TSH level is suppressed, seeming to indicate excessive levels of thyroid hormones, despite the fact that the measured t4, free t4 and t3 levels are in the normal range. Recent studies have demonstrated that some of these patients have isolated elevations in free t3 levels. Thus this test is possible useful to confirm very minimal hyperthyroidism in patients with subnormal serum TSH concentrations. It should not be used if the TSH is normal.