Many authorities now advocate that the first-line assessment of thyroid function should be measurement of
thyrotropin (TSH). The latest serum TSH assays (third generation) are more sensitive than the second generation but the
reagents are more costly. We have examined whether overall assay
reagent costs would be higher or lower with a third-generation assay, in a laboratory that serves a population of almost 500,000. In a prospective study over six weeks, 505 samples with a second-generation serum TSH less than 0.5 mU/L (303 for screening and 202 for monitoring
thyroxine therapy) had an additional third-generation TSH analysis. With a second-generation assay for screening, 11% more free
thyroxine (FT4) measurements were required to exclude
thyrotoxicosis but there was a 42% saving on the
reagent budget compared with a third-generation assay. In patients taking
thyroxine, 33% more FT4 measurements were required to exclude over-replacement but the calculated saving in
reagent costs was 53%. The costs of all other aspects of the two methods were similar. In this community-based sample, the improvement in sensitivity yielded by the third-generation assay at the lower end of the normal range reduced the number of confirmatory FT4 levels required to exclude
thyrotoxicosis or over-replacement with
thyroxine, but
reagent costs were nevertheless higher than for second-generation assays. In financial terms, there is little justification for use of assays with sensitivity greater than the second generation (0.1 mU/L).