Using rT3 to Assess Hypothyroidism
Reverse T3 (rT3): Should it be measured in patients with hypothyroidism.
There is a lot of controversy about whether reverse T3 (rT3) should be measured to assess hypothyroidism. The normal thyroid gland secretes T4 (an inactive precursor), T3 (the active hormone), and reverse T3, a biologically inactive form of T3 that may block T3 from binding to the thyroid hormone receptor. Most of the circulating T3 is derived from 5′-deiodination of circulating T4 in the peripheral tissues by type 1 deiodinase. Deiodination of T4 can occur at the outer ring (5′- deiodination), producing T3 (3,5,3′-triiodothyronine), or at the inner ring, producing reverse T3 (3,3,5′-triiodothyronine) by type 3 deiodinase. Type 2 deiodinase in the pituitary also converts T4 to T3 and is regulated differently from type 1 deiodinase.
Many years ago, endocrinologists realized that in severe illnesses, type 3 deiodinase increases and rT3 is often high and T3 is often low. Endocrinologists termed this “sick euthyroid syndrome” and noted that it was common in many types of chronic illnesses, especially in patients hospitalized in intensive care units. It was usually recommended that these patients not be treated with thyroid hormone and in most cases, the elevated rT3 resolved when the patients health returned.
However, more recently, more alternative doctors, including doctors who used to be known as antiaging doctors and now are called functional medicine doctors, have made quite an issue about rT3. They argue somewhat appropriately that high rT3 is bad and can block T3 from binding to the thyroid hormone receptor, They also state that rT3 can go up in various conditions including systemic illness, stress, inflammation, chronic pain, dieting, weight gain, and depression. They often quote articles showing that these diseases are correlated with a high rT3, but do not necessarily show that the rT3 plays a significant role in the disease. These functional medicine doctors rely extremely heavily on rT3 to treat patients that may have no other laboratory findings of hypothyroidism and often prescribe them T3-only preparations to try to lower the rT3. They have not published on whether this is effective or not, and may treat a person with completely normal thyroid function tests and put him on T3 to try to lower the rT3. They often use a high rT3: T3 ratio as further indication to treat these patients.
Dr. Friedman has seen many of these patients put on thyroid hormone, especially T3, inappropriately. T3 does not penetrate the brain well, not nearly as well as T4 does, and these patients often suffer from hyperthyroidism systemically and hypothyroidism in their brain. Dr. Friedman also believes that a healthy thyroid works best on its own and prescribing exogenous thyroid hormones to a patient with a normal thyroid gland, causes the gland to stop making its own hormones. Sometimes it is very hard for the patient to stop thyroid hormones, especially if given in high enough dose or for a long period of time. Therefore, he is quite cautious about who he puts on thyroid medicine and who he does not.
Dr. Friedman decided to measure rT3 in 100 consecutive patients who came to see him with potential thyroid problems. All of these had signs and symptoms of hypothyroidism, and many of them were already treated with different thyroid preparations. He used the upper limit of normal for rT3 at either Quest or Labcorp, which is usually 24 ng/dL. He did not calculate any type of ratios. He found that 18 of the 100 patients had a rT3 above the normal range. Three of these patients were not on any thyroid medicine, and 2 of them, when the rT3 was repeated, became normal. One of them was on a T3/T4 combination, 2 of them were on desiccated thyroid alone, 4 of them were on desiccated thyroid plus T4, and 8 of them were on T4 alone. The 8 patients on T4 was a relatively high percentage of the people on T4 alone.
Dr. Friedman concluded from his study that measuring rT3 may be helpful in a small number of patients, especially those already on thyroid treatments that contain T4. Most patients that do have an elevated rT3 on thyroid treatment with either T4 alone or T4 plus desiccated thyroid may benefit from decreasing the amount of T4 given and increasing the amount of T3 or the desiccated thyroid given. Based on this data, he would not recommend measuring rT3 in most patients who are not taking thyroid medicine, as only a very small percentage of them had an elevated rT3.
The concept is in those patients with a high rT3 you can give them either T3 alone or T3 that is part of desiccated thyroid to “kick out the rT3.” This would lead to improved thyroid function and symptom improvement.
The area of rT3 is clearly an interesting area, and there is a clear division between endocrinologists who do not measure all and the functional doctors who measure it in almost everybody and treat almost everybody for it. Dr. Friedman is hoping to get IRB approval and write up a peer-reviewed article measuring rT3. Dr. Friedman feels that the test can be helpful in certain patients, but some discretion needs to be used in who gets thyroid medicine (especially for the rest of their life) and who does not.
Dr. Friedman is an expert at putting the right patients on the right thyroid hormone. For more information about Dr. Friedman’s practice or to schedule an appointment, go to www.goodhormonehealth.com or email us at firstname.lastname@example.org.