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Selective Performance of the T₃ Uptake: A Practical Approach to Thyroid Function Studies. Concise Communication

William Beaumont Army Medical Center, El Paso, Texas

Correspondence: For reprints contact: Douglas L. Daniels, Nuclear Medicine Service, William Beaumont Army Medical Center, El Paso, TX 79920.

ABSTRACT

Of 5008 patients with normal serum thyroxine concentrations (T₄ = 4.8–12.2 µg/dl) assayed by our laboratory during an 11-mo period, only 41 (0.82%) had an abnormal free thyroxine index (FTI). Narrowing the T₄ normal range by 0.5 µg/dl at both ends to create a discriminatory range of 5.3–11.7 µg/dl reduced this number to 13( 0.26%). Evaluation of the medical records for 11 of these 13 patients revealed histories of estrogen and thyroid-hormone administration, individually or in combination, which may have contributed to their divergence from the population. A program for the selective performance of serum T₃ uptake (T₃U) and FTI determinations only on those sera with T₄ concentrations outside of a discriminatory range is described and evaluated. This initial discrimination is the first step in a practical system for selective performance of additional in vitro thyroid function assays to derive maximum diagnostic information from a single patient sample. In our institution the program described is easily adapted to computerization and has resulted in a substantial reduction of reagent costs while providing more efficient utilization of laboratory personnel.