DOES AN INCREASE IN THE SENSITIVITY OF SERUM THYROTROPIN ASSAYS REDUCE DIAGNOSTIC COSTS FOR THYROID-DISEASE IN THE COMMUNITY

Many authorities now advocate that the first-line assessment of thyroi d function should be measurement of thyrotropin (TSH). The latest seru m TSH assays (third generation) are more sensitive than the second gen eration but the reagents are more costly. We have examined whether ove rall assay reagent costs would be higher or lower with a third-generat ion assay, in a laboratory that serves a population of almost 500 000. In a prospective study over six weeks, 505 samples with a second-gene ration serum TSH less than 0.5 mU/L (303 for screening and 202 for mon itoring thyroxine therapy) had an additional third-generation TSH anal ysis. With a second-generation assay for screening, 11% more free thyr oxine (FT4) measurements were required to exclude thyrotoxicosis but t here was a 42% saving on the reagent budget compared with a third-gene ration 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 meth ods were similar. In this community-based sample, the improvement in s ensitivity yielded by the third-generation assay at the lower end of t he normal range reduced the number of confirmatory FT4 levels required to exclude thyrotoxicosis or over-replacement with thyroxine, but rea gent costs were nevertheless higher than for second-generation assays. In financial terms, there is little justification for use of assays w ith sensitivity greater than the second generation (0.1 mU/L).