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Acute Effects of Amiodarone Administration on Thyroid Function in Patients with Cardiac Arrhythmia

Laboratory of Cardiovascular Endocrinology, National Research Council Institute of Clinical Physiology (G.I., Al.C., R.B., C.M., S.B., M.R., C.P., An.C. A.B.), Pisa, Italy; and Division of Endocrinology, Department of Medicine (I.C.), University of California at Los Angeles School of Medicine, Los Angeles, California 90026-1682

Address all correspondence and requests for reprints to: G. Iervasi, National Research Council Institute of Clinical Physiology, Via Savi 8, 56100 Pisa, Italy.

Because little has been published on early effects of treatment with amiodarone on thyroid function, we studied serum total and free thyroid hormone, reverse T₃, and TSH levels in patients with cardiac arrhythmias during the first 10 days of treatment with a loading dose of amiodarone by iv infusion. Twenty-four patients were enrolled in the study. A standardized loading regimen for the iv infusion of amiodarone was used. The protocol provided the iv infusion of 20 mg/kg per day on day 1, the iv infusion of 10 mg/kg per day on day 2, then 600 mg/day per os for 7–10 days, and finally, in patients chronically treated with the drug, the dose was gradually reduced to 400–200 mg/day per os. Total and free concentrations of T₄ tended to progressively and significantly increase (P < 0.0001 repeated measures ANOVA) starting from the fourth day of therapy, whereas total T₃ decreased from the second day progressively (P < 0.0001) throughout the study; free T₃ did not significantly change. TSH levels early and significantly (P < 0.001, by ANOVA) increased throughout the study, starting from the first day of therapy and reaching at 10 days a value 2.7 times higher than the basal value. Reverse T₃ levels progressively and significantly (after 2 days of treatment) increased and paralleled the TSH values, reaching at the 10th day a value about 2 times higher than basal value. In conclusion, our data suggest that after iv treatment with amiodarone: 1) TSH is the first hormone to change significantly followed by reverse T₃, T₄, and T₃; 2) the progressive fall of T₃ levels reflects an inhibition of the peripheral conversion of T₄ to T₃; 3) the observed later increase of total and free T₄ levels may be explained by a contribution of direct thyroidal stimulation by TSH and/or by a reduction in T₄ clearance.

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