The immunofluorometric TSH assay as first-line test for suspected thyroid dysfunction

In this study of 103 patients with suspected thyroid dysfunction, the diagnostic value of a single basal immunofluorometric (IFMA) TSH measurement was evaluated and compared with the classical TRH test with RIA-TSH measurements, plasma TT4 concentrations and FT4I. A single basal TSH determination accurately predicted the TRH-stimulated TSH response, making the TRH test redundant in most patients. Because undetectable basal TSH did not always exclude a small rise in TSH, the TRH test could still be indicated in patients receiving thyroxine suppression therapy for thyroid cancer. Basal TSH differentiated accurately between euthyroidism and thyroid dysfunction, especially at the decision values of 0.20 and 4.0 mU/l, as proposed in this study. For diagnosis of clinical and subclinical hypo- and hyperthyroidism, additional measurement of TT4 and/or FT4I is necessary. A 2-yr follow-up of patients with subclinical thyroid dysfunction did not show progression to clinical disease. In some of the patients with subclinical hypothyroidism, substitution therapy with thyroxine had been started after initial testing. Indications for treatment of subclinical thyroid dysfunction are discussed.