A strategy for testing thyroid function with discretionary assay of thyroid stimulating hormone (TSH) following initial measurement of total plasma thyroxine (tT4) was retrospectively evaluated in relation to the diagnosis of primary hypothyroidism. Over a two year period 14 641 tT4 assays were done and 6887 TSH assays, of which 29% were initiated in the laboratory. The percentage of raised TSH values (over 5 mU/1) was similar to those for clinician and pathologist initiated requests (23.2% and 23.6%). Some TSH requests (1620) were cancelled; when 212 of these were subsequently analysed the TSH value was raised in 5.7% The incidence of raised TSH values in 188 patients with a tT4 below 100 nmol/1 (7.8 micrograms/100 ml), in whom TSH was not requested, was 5.3% Most of these raised TSH results could be readily explained by information available to the requesting clinician but not given on the request form.
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