Throughout my twenty-year neuroendocrine practice I have seen many patients who presented clinically with symptoms and signs of hypothyroidism, which is deficiency of thyroid hormone effects. Yet, their thyroid hormone levels were not abnormally low when measured. This made me wonder if they had Reverse T3 Syndrome.
Thyroid hormone comes in two main forms. L-thyroxine is the main form secreted from the thyroid gland. L-thyroxine is nicknamed “T4” since each L-thyroxine molecule has 4 iodine atoms attached to it. When T4 travels through the bloodstream and enters a cell, one of the iodine atoms is cleaved off it. This then leaves only 3 iodine atoms and the result is called triiodothyronine, or T3.
T3 is the more active hormone. T3 is 7-10 times more potent than T4. T4 can be thought of as a proto- or precursor hormone, and T3 the active hormone. However, a different process can, and does, occur when an opposite, “mirror-image,” iodine is cleaved from T4 and the result is called reverse T3, or RT3.
RT3 is interesting because it blocks T3 from its functioning. RT3 can be thought of as the thyroid system putting on its brakes. Normally T3 revs body functions. But, there are times when the body should not be revved, like when we are critically ill. Several other conditions also are known to lead to increased production of RT3, including stress, severe dieting, low serum iron levels, cortisol deficiency and diabetes.
Measuring RT3 along with T3 and T4 will help in these situations. It is known that the ratio of T3 to RT3 should be >10. There should be at least 10-times as much T3 as RT3 in our bloodstreams. When I have measured these hormone in the patients described above I most often find their T3/RT3 ratios to be <10. I refer to this as the RT3 Syndrome. Raising their T3/RT3 ratios to >10 makes their low thyroid symptoms and signs go away.
The important points are first to see if any of the above causes of high RT3 are present. However, if none are in play then the treatment to increase the T3/RT3 ratio must be with T3 only. Treatment with T4 will not help. This is because RT3 can only be made from T4, not from T3. So treatment with T4 is a losing proposition as too much is continually made into RT3. T3 from a pharmacy is called liothyronine. A common brand is called Cytomel. Treatment with normal doses of liothyronine (Cytomel) will do the trick.