T4 suppression test involving 24-hour thyroidal 131I uptake in patients with Graves' disease compared to the T3 suppression test.

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  • KOBAYASHI ISAO
    Division of Endocrinology, Department of Medicine, Gunma University School of Medicine
  • ISHII HIDEKAZU
    First Department of Internal Medicine, Gunma University School of Medicine
  • MORI MASATOMO
    Division of Endocrinology, Department of Medicine, Gunma University School of Medicine
  • KOBAYASHI SETSUO
    First Department of Internal Medicine, Gunma University School of Medicine

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A T4 suppression test involving 24-h thyroidal 131I uptake was carried out on patients with Graves' disease during therapy with an antithyroid drug.Thirty-three patients received propyithiouracil (PTU) for at least 1 year. Each patient was given 75 μg L-T3 daily for 8 days in conjunction with PTU (50 mg/day) at the time of the experiment and then the 24-h thyroidal 131I uptake (post T3 uptake was measured).Twenty-two patients had normal levels of serum T3 and T4-I before L-T3 administration and were divided into 2 groups, positive T3 suppression (post T3 uptake ≤ 35%, group I) and negative T3 suppression (post T3 uptake> 35%, group II).Eleven patients showed elevated serum T3 or T4-I concentrations before L-T3 administration (group III).The T4 suppression test was then performed on the same patients.Each patient was given 300 μg L-T4 daily for 8 days in conjunction with PTU and the 24-h thyroidal 131I uptake (post T4 uptake) was measured.Some patients receiving PTU (50 mg/day) were switched to MMI (5 mg/day) and the T4 suppression test was done one month later.A significant correlation between the two suppression tests during PTU therapy was observed (r 0.961, p<0.01).None of the patients complained of side effects during the T4 suppression test.The mean post T4 uptake in group I was 18.8±3.3% during PTU therapy and 5.4±1.3% during MMI therapy.The mean post T4 uptake in group III was 53.5±.7.0% during PTU therapy and 16.4±5.8% during MMI therapy.<BR>The present findings indicate that the T4 suppression test with the lower toxicity reported herein is equivalent to the conventional T3 suppression test.However, it appears to be difficult to evaluate the results of the T4 suppression test on the basis of the 24-h uptake during MMI therapy.

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