Selective performance of the T3 uptake; a practical approach to thyroid function studies. Concise communication.

Of 5008 patients with normal serum thyroxine concentrations (T4 = 4.8-12.2 micrograms/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 microgram/dl at both ends to create a discriminatory range of 5.3-11.7 microgram/dl reduced this number to 13 (0.26%). Evaluation of the medical records for 11 of these 13 patients revealed his histories of estrogen and thyroid-hormone administration, individually or in combination, which may have contributed to their divergence form the population. A program for these selective performance of serum Tc uptake (T3U and FTI determinations only on those sera wtih T4 concentrations outside of a discriminatory range is described and evaluated. This initial discrimination is the first step in 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 of 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.