Reevaluation of serum TSH determination in various pituitary disorders using highly sensitive immunoradiometric assay

Abstract

In thirty-one patients with various pituitary disorders, their serum TSH levels were reevaluated using two kinds of highly sensitive immunoradiometric assays (IRMAs). TSH levels were measured by radioimmunoassay (RIA) supplied by Daiichi Radioisotope Laboratory and the IRMAs, which were RIA-gnost TSH Ultrasensitive and SUCROSEP TSH IRMA using two types of monoclonal antibodies. The sensitivities of them were 0.08 and 0.1 microU/ml, respectively. Free thyroxine and free triiodothyronine levels were measured by RIAs (Amerlex kits). There was a significant correlation between basal TSH levels and the maximum increase in serum TSH above baseline using the RIA and the IRMAs when responses to TRH (500 micrograms i.v.) was determined, but no correlation between basal TSH and thyroid free hormone levels. In their TRH tests, three patients showed absent responses to TRH which were measured by the RIA, although by the IRMAs slight responses were observed. In two patients, exaggerated responses to TRH were observed and the increase of TSH levels measured by the IRMAs showed 143 approximately 210% higher than that of the RIA. The IRMAs were able to handle their peak levels time in all the patients with responses, however, in some patients the RIA was not sensitive enough to distinguish it. These results indicated that measurement of TSH levels by the IRMAs might be useful in the evaluation of the TSH secretion capacity in patients with various pituitary disorders.