TSH determination by immunoradiometric assay (IRMA) has enabled the detection of subnormal
levels of TSH and has thus opened up new vistas in the differentiation between euthyroidism
and hyperthyroidism. The present investigations have proved that in patients with
autonomous functioning thyroid adenoma with progressive clinical trend, when 'non-toxic'
nodular goitre transforms into toxic adenoma, the basal serum TSH level is gradually
decreasing. In case of a functioning thyroid adenoma detected by scintigraphy, the
basal TSH concentration of the serum over 0.3 mU/l indicates euthyroidism. In preclinical
hyperthyroidism, similarly to toxic adenoma, the serum thyroid hormone determination
contributes to differentiating between the two clinical states. The IRMA for TSH determination
makes it unnecessary, in the majority of cases, to perform the TRH (thyreotrop-releasing
hormone) loading-test, which is needed only if the basal serum TSH level is in the
borderline range, i.e. between 0.1 and 0.3 mU/l.