Direct Measurement of Serum T4, T3 and rT3	

	Total serum T4, T3, and rT3 (Fig. 15.12) are conveniently measured by widely available commercial radioimmunoassays (RIAs). Since serum T4 is largely protein bound and does not correlate with free T4 if there are binding protein abnormalities, T4 is useful as a screening agent but is not reliable as a diagnostic test. Serum total T3 is less affected by binding protein changes and is useful, particularly in diagnosis of thyrotoxicosis with borderline T4 values. Serum rT3 may be measured as an indicator of the ‘euthyroid sick’ syndrome in patients with low serum T4 or T3. Measurement of protein-bound iodine (PBI) (used in past years) correlates with serum T4 if no other organic iodine is present. It also can detect abnormal iodinated proteins. The test is not currently used in practice.
See also:
• Thyroid Hormone Transport.