To distinguish low T3 "sick thyroid " � patients (usually increased) from true hypothyroidism
Interpretation
Increased In
Severe nonthyroidal illness except in some liver disorders, HIV, renal failure
Usually in hyperthyroidism and increased serum TBG
Decreased In
Often in hypothyroidism but overlaps with normal range
Limitations
Measurement is occasionally useful in hospitalized patients to distinguish between nonthyroidal illness and central hypothyroidism. Values are lower in central hypothyroidism, because of low production of T4. In patients with mild hypothyroidism, rT3 levels may be normal or even slightly higher limiting its usefulness.