Sensitive thyrotropin and free thyroxine testing in outpatients. Are both necessary?

BACKGROUND

The appropriate use of specific thyroid function tests to detect thyroid dysfunction remains controversial; some experts recommend both a sensitive thyrotropin (sTSH) test and a free thyroxine (FT4) test, while others recommend an sTSH test alone.

OBJECTIVE

To determine how often sTSH and FT4 tests are ordered simultaneously, how often the results are discordant, and under what circumstances a single test of thyroid function may be sufficient to rule out thyroid dysfunction.

METHODS

Retrospective descriptive study of all sTSH and FT4 tests performed on adult outpatients during a 6-month period. If both sTSH and FT4 tests were performed on a single serum specimen, the results were classified as concordant (both tests indicating hypothyroidism, hyperthyroidism, or euthyroidism) or discordant. Chart review was performed on patients with normal sTSH results and abnormal FT4 results.

RESULTS

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 specimens (48% and 89% of the total number of sTSH and FT4 tests ordered, respectively) from 2629 patients. Of the sTSH results, 69.8% were within 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 specimens 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% confidence interval, 0.8%-1.8%). Chart review did not disclose any instances when an abnormal FT4 results contributed to the treatment of an individual 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 result is normal; at our institution this approach would obviate the need for at least half of the FT4 tests performed each year.