Reverse triiodothyronine

Low T3 syndrome (euthyroid sick syndrome, non-thyroidal illness) is characterised by a disturbed homeostasis of thyroid hormones. This results in an accumulation of the inactive metabolite rT3, a normal to low TSH level, a low FT3 level and, with a long disease duration, a low FT4 level. The determination of rT3 can be useful to differentiate between patients with central hypothyroidism and patients with low T3 syndrome.