Reverse T3 Test

This blood test is used to help determine if an individual has "nonthyroidal illness syndrome" or "sick euthyroid syndrome" as opposed to hypothyroidism.

Reverse triiodothyronine (rT3) is an isomer of triiodothyronine (T3) that typically demonstrates no biological activity. Reverse T3 is biologically inactive and does not stimulate thyroid hormone receptors.

However, thyroid hormone levels are commonly observed secondary to acute (eg, septic shock, myocardial infarction) or chronic (eg, advanced AIDS) systemic nonthyroidal illnesses. The hallmark features of this "nonthyroidal illness syndrome" are a low T3 levels accompanied by an increase in rT3 levels. Diminished T3 levels are thought to reflect altered thyroid homeostasis as a mechanism of adapting to severe illness.