Thyroid function testing in outpatients: are both sensitive thyrotropin (sTSH) and free thyroxine (FT4) necessary?

Despite improved thyroid function testing assays, appropriate use of these commonly ordered tests to detect thyroid dysfunction remains controversial. This study determined if a normal sensitive thyroid stimulating hormone (sTSH) test alone is sufficient to rule out thyroid dysfunction in outpatients.

This was a retrospective analysis of initial sTSH and free thyroxine index (FT4) tests ordered during a 26-month period. Test results were classified as concordant if both the sTSH and FT4 indicated the same findings (ie, euthyroid, hyperthyroid, or hypothyroid). The results were classified as discordant if the sTSH and FT4 did not indicate the same findings.

There were 1,392 paired sTSH and FT4 results. Of 1,340 results classified as concordant (96.2%), 1,187 specimens were consistent with euthyroidism, 41 with hyperthyroidism, and 112 with hypothyroidism. Of the remaining 52 (3.8%) discordant results, 47 met the definition of subclinical thyroid dysfunction. Excluding these 47 results yielded a concordance rate of 99.6%. Of the 1,192 normal sTSH results, FT4 was low in two and high in three. If FT4 tests had not been ordered on the 1,192 specimens with normal sTSH levels, the savings over the study period would have been more than dollars 3,360.

If the sTSH is normal, the likelihood of an abnormal FT4 is very small. sTSH alone is adequate to screen outpatients for thyroid dysfunction. Limiting FT4 tests to those with abnormal sTSH results will result in cost savings.