SUN-410 Reverse T(3) in Patients with Hypothyroidism, Helpful or a Waste of Time?

BACKGROUND: The normal thyroid secretes T(4) (an inactive precursor), T(3) (the active hormone) and reverse T(3), a biologically inactive form of T(3) that may block T(3) from binding to the thyroid hormone receptor. As about 15% of patients on L-T(4) 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, rT(3) is often high and T(3) is often low and termed this “sick euthyroid syndrome”. However, more recently, alternative or functional doctors have argued that high rT(3) is detrimental and can block T(3) from binding to the thyroid hormone receptor. Without peer-reviewed publications, these functional doctors rely heavily on rT(3) levels to treat patients that may have no other laboratory findings of hypothyroidism and often prescribe them L-T(3)-only preparations to try to lower the rT(3). Hypothesis: Patients on L-T(4) alone will more likely have an elevated rT(3) compared to patients on desiccated thyroid or L-T(4)/L-T(3) therapy. Methods rT(3) 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 rT(3) above the normal range/patients in that category. The cutoff of 24 ng/dL (upper limit of normal for rT(3) at either Quest or LabCorp) is indicated by the line. Overall, 18 of the 98 patients had a rT(3) above the normal range. Patients on L-T(4) alone or desiccated thyroid plus L-T(4) had the highest levels of rT(3) and the highest % above the cut-off. Three of the patients with a high rT(3) were not on any thyroid medicine, and in 2 of them, the rT(3) normalized when repeated. The 8 patients with a high rT(3) on L-T(4) was a relatively high percentage (29%). Conclusion: Measuring rT(3) may be helpful in patients who are already on T(4)-containing thyroid treatments who still have hypothyroid symptoms. Based on this data, measuring rT(3) in most patients who are not taking thyroid medicine is not recommended, as only a very small percentage of them had an elevated rT(3). Future studies are needed to determine if high rT(3) levels correlate with hypothyroid symptoms and if adding L-T(3) or desiccated thyroid to hypothyroid patients on L-T(4) normalizes rT(3) and improves hypothyroid symptoms.