Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1193 page)

   Use

   To distinguish low T
₃
“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 T
₄
. In patients with mild hypothyroidism, rT
₃
levels may be normal or even slightly higher limiting its usefulness.

RHEUMATOID FACTOR (RF)

   Definition

   RF is an immunoglobulin present in the serum of 50–95% of adults with RA. It appears in serum and synovial fluid several months after onset of RA and is present up to years after therapy. The autoantibodies are usually of IgM class, although approximately 15% of RA has IgG class. Most methods detect only the IgM class.

   
Normal range:
<20 IU/mL.

   Use

   Assisting in the diagnosis of RA, especially when clinical diagnosis is difficult