INTERINDIVIDUAL DIFFERENCES IN THE PITUITARY-THYROID AXIS INFLUENCE THE INTERPRETATION OF THYROID-FUNCTION TESTS

OBJECTIVE We investigated interindividual differences in the shape, slope and setpoint of the pituitary-thyroid axis (PTA) in normal persons. Based on these physiological data we propose a novel bivariate concept for the interpretation of thyroid function tests which is less biased by interindividual differences in the PTA than the currently used univariate approach. DESIGN In two separate trials (A and B), healthy volunteers were given small, increasing doses of T3 (7.5-45 mug/day orally) over 5 days. The regulation characteristics of the individual PTAs and the effects of age and gender were assessed by general linear regression models. In addition, serum samples were collected from normal persons to establish the proposed bivariate approach for the interpretation of thyroid function tests. SUBJECTS The regulatory characteristics of the PTA were determined in a total of 21 normal volunteers (eight females, 13 males; age 24-49 years). Single blood samples were collected from 257 normal volunteers. The participants had no history of pituitary or thyroid disease. MEASUREMENTS Free and total thyroid hormone and TSH concentrations were determined in the serum. All samples from one person were analysed in the same assay in duplicate. RESULTS A log-linear relationship between T3 and TSH was found to describe best the individual PTA (multiple r=0.96). However, significant differences in the set-point (P<0.001) and to a lesser degree in the slope (P less-than-or-equal-to 0.05) of the PTA were detected; this variability was not dependent on age or gender. Since these findings invalidate the assumptions on which the current univariate interpretation of thyroid function tests is based, we propose a novel model for the evaluation of thyroid function tests derived from the experimentally determined shape and average slope of the PTA. CONCLUSIONS The presence of significant age and gender-independent interindividual variations in the set-point of the pituitary-thyroid axis raises conceptual problems with the current approach for interpreting thyroid function tests. An easy to use graphical bivariate representation of the normal ranges for thyroid function tests is presented and exemplified by the thyroid hormone and TSH measurements in a large reference population. This concept should improve the diagnostic accuracy in the borderline-normal, and particularly subclinical hypothyroid region of these hormone measurements.