Kent Holtorf, MD: T4 can be either converted to T3, the active hormone that has a metabolic effect, or to reverse T3, which is the inactive form of T3, and actually blocks the effects of T3. Doctors – including endocrinologists – are taught that reverse T3 is just an inactive metabolite, but studies show that it has potent antithyroid effects. In fact, is has been shown to be a more potent inhibitor of thyroid effect than PTU, a medication used for hyperthyroidism. Reverse T3 inversely correlates with intracellular T3 levels, so it is also a marker for tissue hypothyroidism, with higher levels (or lower Free T3/RT3 ratio) indicating a more significant deficiency.
Mary Shomon: Why do you feel reverse T3 plays a role in making it difficult for some thyroid patients to lose weight?
Kent Holtorf, MD: The reverse T3 is produced in times of stress or starvation to reduce metabolism, and with chronic stress or dieting, RT3 can remain elevated, suppressing tissue thyroid activity and metabolism. People on chronic diets – or those who lose significant amounts of weight – will have a lower metabolism than a person with the same weight and muscle mass who had not had not lost significant weight or drastically dieted in the past. This was demonstrated in a study by Leibel published in the journal Metabolism, titled “Diminished Energy Requirements in Reduced-Obese Patients.” This study compared the basal metabolic rate in individuals who had lost significant weight to those of the same weight who had not lost significant weight in the past. The authors found that those who had dieted and lost weight in the past had, on average, a 25% lower metabolism than the control patients who had not lost significant weight.
All those trainers and health gurus that never had a weight problem who tell you to do just as they do don’t realize what a disadvantage it is for people who have had a long-term weight problem. Of course, even these trainers would not even be able to maintain their weight with a metabolism that is 20 to 40% below normal.
We test the resting metabolic rate in our thyroid patients and find it inversely correlates with the reverse T3. The higher the reverse T3, the lower the metabolism, with many such individuals having a metabolism that is 20 to 40% lower than expected for their body mass index (BMI). Nobody believes how little they eat, and they are made to feel like failures – despite doing everything right. Until their metabolic abnormalities are addressed, diet and exercise will certainly fail to achieve long-term success.
Mary Shomon: At what point do you consider reverse T3 too high and requiring treatment?
Kent Holtorf, MD: Like everything else in medicine it is a continuum, but healthy individuals are usually below 250 pg/ml and should have a free T3/reverse T3 ratio greater than 1.8 if free T3 is in ng/dl or 0.018 if free T3 is in pg/ml.
