Warfarin hyperthyroid patients

Describe the reduced efficacy of warfarin in hyperthyroid patients. 

In 5 patients with hyperthyroidism, the response to a single dose of warfarin was decreased 3 months after treatment with iodine-131 (when euthyroid), when compared with before treatment. In addition, the plasma hdf-life and levels of warfarin were higher after treatment. A hyperthyroid patient taking warfarin had a marked increase in his prothrombin times on two occasions when his treatment with thiamazole (methnna-zole) was stopped and he became hyperthyroid again. Another similar case has been described where the required dose of warfarin increased from 35 mg weekly to 65 mg weekly as the patients hyperthyroid state was corrected with thiamazole 30 mg daily. However, the patient then became hypothyroid and required up to 85 mg weekly of warfarin. When the thiamazole dose was withheld for 5 days and then reduced to 5 mg daily, the patient rapidly developed a markedly raised INR of 7 without bleeding complications. In another report, a patient required just 0.5 mg of warfarin daily while hyperthyroid. Another case of possible enhanced response to warfarin in a hyperthyroid patient has been reported. ... 

Patients with hypothyroidism and hyperthjroidism should all be carefully monitored because the response is greatly reduced in hypothyroidism and greatly enhanced in hyperthyroidism (103). The increased warfarin sensitivity is said to be related to increased degradation of clotting factors. When the thyroid disease is treated, the susceptibility of these patients to coumarins gradually normalizes. 

During the hyperthyroid state, other drugs that are metabolized by the liver or eliminated renally may need to be adjusted because metabolism may be increased. Patients using drugs with a narrow therapeutic index such as digoxin, warfarin, and phenytoin should be monitored carefully because dosing adjustments will be necessary as the hyperthyroidism or hypermetabolic state resolves. 

D Amiodarone may decrease warfarin metabolism within a week of coadministration, or the effects of the interaction may be delayed for several weeks. Patients who develop hyperthyroidism secondary to amiodarone may have an additional increased anticoagulant effect, because the turnover of clotting factors is more rapid. If amiodarone is discontinued, effects of the decreased warfarin metabolism may last 1 to 3 months. 

Although the average weekly dose of warfarin is between 30 and 40 mg, some patient-related variables are associated with a lower than usual dose advanced age (>65 years), elevated baseline INR, poor nutritional status, liver disease, hyperthyroidism, and concurrent use of medications known to enhance the effect of warfarin (Table 19-12). Prior to initiating therapy, the clinician should screen for the presence of contraindications to anticoagulation therapy and risk factors for... 

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