Thyroid function testing based on assay of thyroid-stimulating hormone: assessing an algorithm's reliability.

Abstract

To assess the ability of an algorithm for thyroid-function testing (based on assay of thyroid-stimulating hormone [TSH]) to discern euthyroidism in patients with and without conditions affecting thyroid function. The Australian Health Insurance Commission (HIC) specifies clinical categories for which Medicare rebate is given for assay of both TSH and free thyroxine (FT4), but otherwise rebates for thyroid function testing are given for TSH assay only. A prospective study was made of paired TSH and FT4 results of 1000 consecutive assays categorised by indication for testing. An FT4 value within the reference range was accepted as indicating euthyroidism; the reliability of an initial TSH measurement as the sole indicator of thyroid disease was assessed against this criterion standard. A large suburban teaching hospital. Success or failure of the algorithm, with failure defined as an abnormal FT4 level missed because the TSH level was normal. The algorithm failure rate both overall and in the patients not in the HIC clinical categories was 2.7%, and there was no significant difference in algorithm failure rate in the patients in the various HIC clinical categories. The categories and failure rates were: patients being monitored for thyroid disease, 3.4%; patients with the "sick euthyroid" syndrome, nil; patients with psychosis or dementia, 1.1%; patients taking drugs affecting thyroid function, 2.1%; and patients with pituitary dysfunction, one of six cases. The range of FT4 values in patients in whom the algorithm failed was 6.4-29.5 pmol/L in those without thyroid disease and 3.4-27.4 pmol/L in those with thyroid disease. In patients being monitored for thyroid disease, the proportion of abnormal values of TSH alone was significant (P<0.001). We have shown that the HIC's imposition of a TSH-based algorithm by financial fiat is also scientifically acceptable. Use of this algorithm in hospitals (including psychiatric hospitals) will result in substantial savings.