Value and Limitationsof a HighlySensitive lmmunoradiometncAssay for Thyrotropinin the Study of ThyrotrophFunction

Using a highly sensitive and specific immunoradiometric assay for thyrotropin,we studied thyrotrophfunctionin 232 new patients referred to a thyroidclinic and in 13 patients after treatment for hyperthyroidism.Significant thyrotroph responsiveness to thyroliberin (thyrotropin-releasinghor-mone, TRH) was found in all patients with values for basal thyrotropin>0.1 milli-int unit/L. In no overtly hyperthyroid patient was any increment in thyrotropin recorded at 20 mm after thyroliberin administration. In seven patients, four sub-clinicallyhyperthyroidand three who had receivedtreatment, increments in thyrotropinfrom undetectable basal values were recorded, consistentwith incompletethyrotrophsup-pression.By use of assays with even highersensitivity,one may be able to distinguishthese patientsfrom overtlyhyper-thyroidpatients. Addftlonal Keyphrases: thyroidstatus thyroliberin A recently introduced immunoradiometric assay (ina) for thyrotropin C1’SH) is about 10-fold more sensitive than are most radioimmunoassays (RIAs).4 Clinically it has the advantage that the initially obtained value accurately pre-dicts the TSH (by RIA) response to thyroliberin (TRH), obviating the need for the TRH test (1). The high sensitivity and specificity of the MA should also allow more detailed assessment of thyrotroph function. We were particularly interested to investigate thyrotroph function when basal values for TSH were between the lower detection limits of the na and the RIA (0.1 and 1 milli-int. unit/L, respective-ly), and in the months after treatment of hyperthyroidism, to determine whether detectable basal concentrations of TSH are always associated with thyrotroph responsiveness to TRH, and undetectable concentrations always with unre-sponsiveness. We report our findings in 232 new patient referrals and in 13 patients after treatment of hyperthyroid-ism