LOW RESPONSE OF THE THYROID-GLAND TO ENDOGENOUS THYROTROPIN INCREASEDBY THYROTROPIN-RELEASING-HORMONE IN PATIENTS WITH EUTHYROID GRAVES-DISEASE

Euthyroid Graves' disease is defined as Graves' ophthalmopathy without hyperthyroidism, and the thyroid-stimulating antibody (TSAb) has been known to be a good marker for diagnosis. However, the question of why TSAb does not cause hyperthyroidism arises. To settle this, we examin ed thyroid responsiveness to endogenous thyrotropin (TSH) increased by thyrotropin-releasing hormone (TRH) in 23 patients with euthyroid Gra ves' disease. Nineteen patients (83%) had positive TSAb and 21 (91%) h ad at least one of the autoantibodies to the thyroid gland. Only one p atient (4%) had positive thyroid-stimulation blocking antibody (TSBAb) . Basal levels of free thyroxine (FT4), free triiodothyronine (FT3), a nd thyrotropin (TSH) in patients were not different from those in age- and sex-matched normal controls (n = 25). Response of TSH to TRH was normal; however, an increase of FT3, either absolute or as a multiple of baseline, in the TRH test in these patients (0.46 +/- 0.23 pg/mL, P < 0.001; 1.14 +/- 0.09 fold, p < 0.001) was significantly lower than that in controls (0.86 +/- 0.19 pg/ml; 1.26 +/- 0.07 fold). There was no correlation between the Delta FT3/Delta TSH ratio and TSAb activity . It is concluded that thyroid responsiveness in euthyroid Graves' dis ease is lower than in normal controls and this explains the euthyroid function in the patients even in the presence of TSAb.