Quantification of the negative feedback mechanism between pituitary and thyroid in subjects with normal thyroid function

Using sufficiently sensitive and precise assays, we systematically investigated the correlation between thyrotropin, thyroglobulin, index of free thyroxine and index of free triiodothyronine after different doses of thyroxine (25, 50, 100, 150 μg), which were administered daily for 10 days to individuals with normal thyroid function and in a control group. Analysis of the data using relative median values expressing changes to basal values before administration of thyroxine yielded the following results : (i) thyrotropin and thyroglobulin decreased monoexponentially, depending on the doses of thyroxine administered : (ii) the extent of their decrease showed a linear correlation with the dose of thyroxine administered and was greater for thyrotropin than for thyroglobulin ; (iii) the relative velocity of their decrease increased monoexponentially with the dose of thyroxine and did not differ between thyrotropin and thyroglobulin. These results provide strong evidence for a clear quantitative reaction of the feedback mechanism and confirm that the secretion of thyroglobulin is a physiological process dependent on thyrotropin. The high intra-individual variations obtained for thyrotropin were probably due to its pulsatile secretion.