Non-linear correlation of basal serum TSH and TRH-stimulated TSH response: new aspects in pituitary-thyroid...

The relationship between basal and TRH-induced TSH in the 200 micrograms i.v. TRH test has been reinvestigated using one of the new 'supersensitive' TSH-IRMA assays. A total of 761 consecutive TRH tests (1522 sera) in unselected patients attending the Thyroid Out-patient Department was studied. The closest correlation (r = 0.962, In S.D. = 0.46787) of both TRH test parameters was found using logarithmic transformation of TSH-levels and a least squares fit based on a sixth degree polynomial approach. The resulting correlation curve revealed three points of inflection, one located in the subnormal, one in the normal and one in the high-normal range of the basal as well as the TRH-induced TSH concentrations. This indicated that 'fine-tuning' of pituitary-thyroid axis occurs according to a non-linear dose-response relationship. Consequently, the interpretation of the TRH test on the basis of delta-TSH used hitherto has to be reconsidered.