Clinical Studies on Thyroid CT Number in Graves' Disease and Destructive Thyrotoxicosis.

The purpose of the present study was to investigate by a computed tomography (CT) the Hounsfield unit (H.U.) of the thyroid in hyperthyroid and euthyroid Graves' disease and destructive thyrotoxicosis.The mean thyroid CT number in 95 controls was 122±18 H.U. (±SD) and did not change significantly with advancing age. The mean thyroid CT number (±SD) of 85±22 H.U. in 60 patients with hyperthyroid Graves' disease was significantly (P <0.001) lower than either in normal controls or 116± 22 H.U. in 11 patients with euthyroid Graves' disease (P<0.001). Comparison of thyroid hormones and TSH receptor Ab values of untreated patients with a normal and an abnormally low thyroid CT number showed that serum total and free T3 were significantly (P <0.05) higher in the latter group than in the former group. With respect to the effect of methimazol (MMI) on the thyroid CT number, in the untreated 10 patients with a low thyroid CT number, the initial mean CT number was 65±11 H.U. and increased significantly (P<0.05) to 76± 14 H.U. after treatment with MMI. In contrast, in 6 patients with a normal thyroid CT number prior to therapy, the initial mean thyroid CT number was 102±11 H.U. and fell significantly (P<0.05) to 84±16 H.U. after treatment with MMI.The thyroid CT number in destructive thyrotoxicosis is markedly decreased to less than 70 H.U. and the mean values of 57±7 H.U. in 6 patients with silent thyroiditis and of 61±5 H.U. in 7 with subacute thyroiditis differ significantly (P<0.001) from Graves' disease. In conclusion, the thyroid CT number is significantly reduced in hyperthyroid Graves' disease, normal in euthyroid Graves' disease and markedly decreased in destructive thyrotoxicosis. The high T3 value seemed to play an important role in the pathogenesis of a decline in the thyroid CT number in Graves' disease. An antithyroid drug therapy caused two different changes in the thyroid CT number, depending on whether the thyroid CT number prior to therapy was normal or low.