FREE THYROID-HORMONES AND A 3RD-GENERATION TSH ASSAY IN THE DETECTIONOF HYPERTHYROIDISM DURING LONG-TERM THYROXINE TREATMENT IN THYROID-CARCINOMA PATIENTS

We evaluated the value of serum-free thyroid hormone and thyrotropin ( TSH) concentrations in the detection of peripheral hyperthyroidism dur ing thyroxine suppression therapy. A total of 57 patients on a stable thyroxine dose and 70 controls participated in the study. Serum-free t hyroxine (FT4), free triiodothyronine (FT3) and TSH were measured by i mmunoassays based on time-resolved fluorescence (Delfia(R)). The assay for TSH was a modification of a third generation Delfia hTSH Ultra me thod. The patients were classified into euthyroid and hyperthyroid sub groups based on clinical signs and symptoms (Wayne index). Systolic ti me intervals (STI) were measured. The Wayne indices were higher among patients than controls (p < 0.0001). The STI results were similar in p atients and controls. Only FT4 had the discriminatory power for classi fying euthyroid and hyperthyroid patients according to discriminant an alyses. The diagnostic value of FT4 was further assessed by calculatin g the area under the relative operating characteristic (ROC) curve. Th e area was 0.707 (SE 0.0918), which was significantly different from a n area of 0.5, i.e. the area of a test of no value (p = 0.032). In con clusion, a high serum FT4 concentration indicates hyperthyroidism duri ng long-term thyroxine treatment among thyroid carcinoma patients. Alt hough the degree of TSH suppression can now be exactly monitored with new third generation TSH assays, hyperthyroidism cannot be defined usi ng TSH concentration in thyroid carcinoma patients. Therefore, additio nal serum FT4 concentration assays are needed in the assessment of hyp erthyroidism associated with TSH suppression therapy in thyroid carcin oma patients.