CLINICAL USEFULNESS OF A HIGHLY SENSITIVE ENZYME‐IMMUNOASSAY OF TSH

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Abstract
Using a recently developed sensitive enzyme immunoassay (EIA) for TSH, serum TSH levels in normal subjects and patients with various thyroid disorders were measured. The minimum detectable concentration was less than 0.005 mU/l. An excellent correlation was observed between TSH values measured by EIA and by sensitive radioimmunoassay (r= 0.932). In 26 normal subjects, serum TSH ranged from 0.30 to 2‐65 mU/l (geometric mean 0.97 raU/l). In 27 untreated hyperthyroid Graves' patients, serum TSH was detected in all but one, ranging from 0.005 to 0.19 mU/l (geometric mean 0.040). These values were proved to be specific for human TSH by absorption, recovery, cross‐reactivity and dilution studies; non‐specific serum effects were also ruled out. In 10 patients with destructive thyroiditis, similar TSH values (0.005‐0.20 mU/l, mean 0.032) were observed. However, no significant correlations between TSH and circulating thyroid hormone levels were observed in these thyrotoxic conditions. Furthermore, 9 untreated Graves' patients did not respond to a single i.v. injection of TRH. In 13 hypothyroid patients with hypothalamopituitary disorders, 10 had basal TSH levels of less than 0.4 mU/l, and TRH tests in five gave peak TSH values of more than 0.8 mU/l, which were associated with significant increases in serum T3. In three patients with Sheehan's syndrome, elevated basal TSH levels were observed. Two of them responded to TRH, but these TSH elevations were not associated with T3 increases. In conclusion, this sensitive EIA could measure TSH levels in most patients with thyrotoxicosis and central hypothyroidism. However, the true significance of these measured values needs to be elucidated by future studies. The conventional radioimmunoassay (RIA) for TSH cannot fully discriminate low TSH levels from those of euthyroid subjects, but recently developed sensitive RIA and immunoradiometric assay (IRMA) have been reported to be able to discriminate between them (Pekary et al., 1975; Smith et al., 1979; Mori et al., 1983; Seth et al., 1984). However, even these assays cannot detect serum TSH in patients with thyrotoxicosis. Detectable amounts of TSH have been observed in the pituitary of hyperthyroid Graves' patients (Kumahara et al., 1967; Mori, 1973). We could expect them, therefore, to secrete some TSH into the circulation, but to detect low TSH levels, it is necessary to apply a very sensitive assay. Cytochemical bioassay of TSH has been reported to be of sufficient sensitivity (Bitensky et al., 1974). However, the assay is not suitable for routine use, and is known to be interfered with by certain non‐specific serum effects (Condliffe & Weintraub, 1979). We have recently reported the development of a highly sensitive enzyme immunoassay (EIA) for TSH which could detect 0.005 mU/l or even less (Inoue et al., 1986). This paper deals with clinical application of the EIA, and discusses the significance of TSH values measured in cases of decreased TSH secretion.

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