SENSITIVE THYROTROPIN AND FREE-THYROXINE TESTING IN OUTPATIENTS - AREBOTH NECESSARY

Background: The appropriate use of specific thyroid function tests to detect thyroid dysfunction remains controversial; some experts recomme nd both a sensitive thyrotropin (sTSH) test and a free thyroxine (FT4) test, while others recommend an sTSH test alone. Objective: To determ ine how often sTSH and FT4 tests are ordered simultaneously, how often the results are discordant, and under what circumstances a single tes t of thyroid function may be sufficient to rule out thyroid dysfunctio n. Methods: Retrospective descriptive study of all sTSH and FT4 tests performed on adult outpatients during a 6-month period. If both sTSH a nd FT4 tests were performed on a single serum specimen, the results we re classified as concordant (both tests indicating hypothyroidism, hyp erthyroidism, or euthyroidism) or discordant. Chart review was perform ed on patients with normal sTSH results and abnormal FT4 results. Resu lts: A total of 6551 sTSH and 3518 FT4 tests were performed during the study period. Both sTSH and FT4 tests were ordered together on 3143 s pecimens (48% and 89% of the total number of sTSH and FT4 tests ordere d, respectively) from 2629 patients. Of the sTSH results, 69.8% were w ithin the normal range, and 92.7% of the FT4 results were normal. The concordance between sTSH and FT4 results was 74.3%. Among the 1835 spe cimens with normal sTSH results, FT4 level was low in 11 patients (0.6 %; 95% confidence interval, 0.3%-0.9%) and high in 24 (1.3%; 95% confi dence interval, 0.8%-1.8%). Chart review did not disclose any instance s when an abnormal FT4 result contributed to the treatment of an indiv idual with a normal sTSH result. Conclusions: The sTSH test alone, and not the combination of sTSH and FT4 tests, should be ordered in most outpatients. An FT4 test should not be routinely ordered if the sTSH r esult is normal; at our institution this approach would obviate the ne ed for at least half of the FT4 tests performed each year.