Clinical Value of a Sensitive Immunoradiometric Assay for TSH

Abstract

SUMMARY.We have investigated a sensitive immunoradiometric assay for serum thyroid-stimulating hormone (TSH) in 45 thyrotoxic patients not on treatment, 464 euthyroid patients and 111 hypothyroid patients on replacement therapy. Fortythree thyrotoxic and seven euthyroid patients were found to have an undetectable TSH (less than 0·2 mIUIL). Previous work has shown a very clear separation of thyrotoxic and euthyroid patients using sensitive TSH assays. However, our extended study here has revealed that a significant number of euthyroid patients with undetectable TSH (1,5% in our study) are likely to be found if TSH becomes the initial test for thyroid function.Thirty of the hypothyroid patients on thyroxine were found to have undetectable TSH, but only one showed clinical signs of thyrotoxicosis. Most of the patients, although having raised serum free T4, had serum free T3 levels within the euthyroid range or just slightly elevated.Most radioimmunoassays for plasma TSH achieve adequate sensitivity for them to be used in the diagnosis and monitoring of treatment of hypothyroidism. However, their sensitivity, normally not less than 1 mUlL, is insufficient for use in investigating thyrotoxic patients.We here describe the use, in clinical practice, of a sensitive immunoradiometric assay (IRMA) capable of detecting 0·2 mUlL of TSH. This assay, a preliminary trial of which has been reported,' has been found valuable in aiding both the diagnosis of thyrotoxicosis and hypothyroidism and for monitoring hypothyroid patients on replacement therapy. Patients and Methods PATIENTSAll patients in this study were classified both clinically and on the basis of routine thyroid function tests. There were 45 thyrotoxic patients, 11 men and 34 women, none of whom were receiving anti-thyroid therapy at the time of blood sampling. The euthyroid population Correspondence: K R Allen, Clinical Biochemistry Department, Wexham Park Hospital, Slough, Berks SL24HL. 506studied consisted of 151 men and 313 women with ages ranging from 16 to 95 years. In addition, we tested 111 hypothyroid patients who were receiving thyroxine. This group included both newly treated patients and patients who had been receiving thyroxine for some years. These doses ranged from 100-200 ug thyroxine daily except for one patient who had been taking 400 J.lg thyroxine daily. Serum from hypothyroid patients with TSH concentrations of less than 0·2 mUlL were assayed for free T3 and free T4. METHODSSerum TSH was measured using an IRMA with magnetic separation of the antibody ('Maiaclone', Serono Diagnostics, Woking, Englandj.? Serum free T4 and free T3 were measured using Amerlex kits (Amersham International). Results SENSITIVITYThe limit of detection of the assay was determined by analysing the zero TSH standard 20 times in this one assay run. 2·5 standard deviations at zero dose was found to be 0·2 mUlL.

Evaluation of the usefulness of a sensitive immunoradiometric assay for thyroid stimulating hormone as a first-line thyroid function test in an unselected patient population

“…The sensitive TSH assay has been proposed as the best single first-line test for thyroid function by many authors [4,5,8,21,22]. However, most previous reports are based on selected groups of thyrotoxic.…”