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An increase in warfarin sensitivity may herald the onset of hyperthyroidism in patients taking amiodarone
The background of the study. Amiodarone
and hyperthyroidism both augment the effects of warfarin, a drug
given to prevent blood clotting. This article described several
patients taking amiodarone and warfarin in whom an increase in sensitivity
to warfarin was the initial sign of amiodarone-induced hyperthyroidism.
Patient 1 was a woman treated with warfarin after heart-valve replacement; her dose of warfarin ranged from 100 to 110 mg per week for many years. After the onset of an irregular cardiac rhythm, she was treated with amiodarone, and a warfarin dose of 75 mg weekly was sufficient to maintain anticoagulation. Fifteen months later, her dose of warfarin had to be reduced to 55 mg weekly. Several weeks later she had onset of fatigue, dyspnea, and palpitations and was found to have hyperthyroidism. Amiodarone was stopped. Seven weeks later her dose of warfarin had to be increased to maintain the desired level of anticoagulation.
Patient 2 was a man with heart failure and cardiac arrhythmia treated with amiodarone and warfarin. Several years later, his dose of warfarin had to be reduced; soon thereafter he was found to have hyperthyroidism. Amiodarone was stopped, and he was treated with prednisone. He subsequently had normal thyroid function, at which time he needed more warfarin.
Patient 3 was a man with valvular heart disease and cardiac arrhythmia treated by valve replacement, amiodarone, and warfarin. Two years later, his dose of warfarin had to be reduced. He then was found to have hyperthyroidism. Amiodarone and warfarin were stopped, and prednisone and methimazole were given. Subsequently, warfarin was reinstituted, at a higher dose than before.
In all three patients the action of warfarin increased, resulting in the risk of bleeding, with the onset of hyperthyroidism.
The conclusions of the study. In patients with a cardiac arrhythmia treated with amiodarone, hyperthyroidism may lead to an increase in the action of warfarin, an action superimposed on the effect of amiodarone itself to increase the action of warfarin.
The original article. Kurnik D, Loebstein R, Farfel Z, Ezra D, Halkin H, Olchovsky D. Complex drug-drug disease interactions between amiodarone, warfarin, and the thyroid gland. Medicine (Baltimore) 2004;83:107-13.
Next Article: Genetic factors are
more important than environmental factors in determining pituitary–thyroid
function in normal subjects
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| Thyroid Digest Index | | | July 2004 Thyroid Digest |
