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Amiodarone Does not Affect the Distribution and Fractional Turnover of Triiodothyronine from the Plasma Pool, but Only Its Generation from Thyroxine in Extrathyroidal Tissues^*

CONICET, Center of Nuclear Medicine, Hospital de Clínicas, University of Buenos Aires 1453 Buenos Aires, Argentina

Address requests for reprints to: Dr. A. A. Zaninovich, Hospital de Clinicas, C.C. 29-Suc. 53, 1453 Buenos Aires, Argentina.

The peripheral metabolism of T₃ has been studied in 10 hypothyroid patients receiving daily oral replacement doses of 200 µg T₄ or 60 µg T₃, in divided doses, before and during treatment with 400 mg amiodarone (AMD) daily for 3 weeks. Each patient received 1 µCi/kg BW [¹²⁵I]T₃, iv. Blood samples were drawn at 10, 20, 30, and 60 min and 2, 4, 6, 8, 12, 24, 36, 48, 60, and 72 h, followed by trichloroacetic acid precipitation and ethanol extraction of the serum samples. The parameters of T₃ kinetics were obtained from the area under the disappearance curve of [¹²⁵I]T₃ from serum using a noncompartmental analysis. In five T₄-treated patients, AMD induced a significant increase in serum T₄ (P < 0.02), serum free T₄ (P < 0.01), and serum TSH (P < 0.02) and a marked decrease in serum T₃ (P < 0.005). Fractional turnover and MCRs were not affected by the drug. However, the extrathyroidal pool of T₃ (P < 0.02) and its degradation rate (P < 0.001) were both significantly diminished. In the five T₃-treated patients, AMD did not induce alterations in the parameters of T₃ kinetics or in serum T₃ and TSH concentrations. The data clearly establish that AMD does not affect the distribution and fractional removal of T₃ from the plasma pool, but only its generation from T₄ in extrathyroidal tissues. This conclusion does not rule out the possibility that the drug may interfere with a number of intracellular events dependent on T₃ action which may be independent of T₃ deiodination.

^* This work was supported by Grant PID-005 from the National Research Council (CONICET) and the Atomic Energy Commission, Argentina.

Received August 17, 1989.

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