We evaluated the SimulTRAC FT4 57Co/TSH 125I dual-isotope assay for the simultaneous measurement of free thyroxin (FT4) by radioimmunoassay analog techniques and of thyrotropin (TSH) by immunoradiometry. Inter- and intra-assay CVs were less than 10% over the entire range tested except for 15.9% at the lowest FT4 concentration. Results obtained by the SimulTRAC assay allowed complete differentiation of 85 hyperthyroid patients and 35 hypothyroid patients from normal subjects. However, such estimations of FT4 or TSH concentrations occasionally were misleading for assessing thyroid status in various clinical conditions. We conclude that the SimulTRAC assay has the same inherent disadvantages possessed by FT4 analog and TSH immunoradiometric assays; however, where results of one of the simultaneous assays may be misleading, the results provided by the other may indicate the underlying pathology without requiring an additional assay.