Applications of a New Chemiluminometric Thyrotropin Assay to Subnormal Measurement*

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Abstract
A new immunochemiluminometric TSH assay (ICMA) was shown to offer improved analytical (+2 SD of zero) and functional (20% interassay coefficient of variation) sensitivity [0.003 vs. 0.045 ± 0.005 (±se; range, 0.01–0.07); 0.018 vs. 0.23 ± 0.02 (range, 0.10–0.35, mU/L); analytical vs. functional sensitivity limit for the ICMA vs. 10 other TSH immunometric assays, respectively]. The ICMA was used to study the physiological relationship between serum TSH and free T4 [as reflected by free T4 index (FT4I)] values at both steady state and 14 days after acute pharmacological T4 administration (3 mg oral T4 load plus 0.3 mg daily). At steady state, an inverse log/linear relationship was found between serum TSH and FT4I values (log TSH = 2.56 − 0.022 FT4I r = 0.84; P < 0.001). Ten to 14 days after acute T4 suppression in 5 euthyroid subjects, serum TSH/FT4I levels had plateaued after decreasing in parallel to the slope of the steady state relationship, suggesting that the degree of T4 suppression of TSH can be predicted from an individual's pituitary TSH/free T4 set-point and the magnitude of the serum T4 elevation achieved. Ambulatory and hospitalized patient sera, previously identified as having low (4 therapy as well as the diagnosis of hyperthyroidism in hospitalized patients with nonthyroidal illness.
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