SUN-410 Reverse T3 in Patients with Hypothyroidism, Helpful or a Waste of Time?

BACKGROUND: The normal thyroid 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. As about 15% of patients on L-T4 replacement with a normalized TSH report continued fatigue and other hypothyroid symptoms, efforts are needed to understand this phenomenon. Decades ago, endocrinologists realized that in severe illnesses, rT3 is often high and T3 is often low and termed this “sick euthyroid syndrome”. However, more recently, alternative or functional doctors have argued that high rT3 is detrimental and can block T3 from binding to the thyroid hormone receptor. Without peer-reviewed publications, these functional doctors rely heavily on rT3 levels to treat patients that may have no other laboratory findings of hypothyroidism and often prescribe them L-T3-only preparations to try to lower the rT3. Hypothesis: Patients on L-T4 alone will more likely have an elevated rT3 compared to patients on desiccated thyroid or L-T4/L-T3 therapy. Methods rT3 was measured in 98 consecutive patients seen in a tertiary Endocrinology clinic with possible or confirmed hypothyroidism (all with severe fatigue) with many of them were already treated with different thyroid preparations. Results: The figure shows the 25%-75% quartiles, ranges and ratio of rT3 above the normal range/patients in that category. The cutoff of 24 ng/dL (upper limit of normal for rT3 at either Quest or LabCorp) is indicated by the line. Overall, 18 of the 98 patients had a rT3 above the normal range. Patients on L-T4 alone or desiccated thyroid plus L-T4 had the highest levels of rT3 and the highest % above the cut-off. Three of the patients with a high rT3 were not on any thyroid medicine, and in 2 of them, the rT3 normalized when repeated. The 8 patients with a high rT3 on L-T4 was a relatively high percentage (29%). Conclusion: Measuring rT3 may be helpful in patients who are already on T4-containing thyroid treatments who still have hypothyroid symptoms. Based on this data, measuring rT3 in most patients who are not taking thyroid medicine is not recommended, as only a very small percentage of them had an elevated rT3. Future studies are needed to determine if high rT3 levels correlate with hypothyroid symptoms and if adding L-T3 or desiccated thyroid to hypothyroid patients on L-T4 normalizes rT3 and improves hypothyroid symptoms.