The effects of subclinical hypothyroidism on warfarin dosage

BACKGROUND: Many patients using warfarin may be at risk of international normalized ratio fluctuations. This may cause severe massive gastrointestinal or cerebral bleeding. Subclinical hypothyroidism in our population is highly prevalent (range from 3.1% to 8.4% for males and females respectively). Overt hypothyroidism diminishes the effect of warfarin but there is a lack of results on the effects of subclinical hypothyroidism on warfarin dosage. METHODS: In this clinical trial, 102 patients using warfarin were checked for TSH, and if the result was abnormal, Serum TSH, T4, T3, and T3RU concentration were assayed again. Mean weekly dosage of warfarin and frequency of its fluctuations were compared 2 months before, 2 months after and also during the treatment period of hypothyroidism. The INR is obtained weekly, to verify dosing within range by stability of the INR. Duration of hypothyroidism treatment was about 3 months. RESULTS: Seventeen (16.6%) patients had thyroid diseases, 15 patients of them (88%) had subclinical hypothyroidism. Weekly dosage of warfarin for the subclinical hypothyroidism state was 33 ± 15 mg (mean ± SD). The weekly dosage of warfarin during the treatment period of hypothyroidism was 30 ± 13 mg. Mean warfarin dosage 2 months after treatment was 29 ± 11 mg. There was a significant difference between warfarin dosage before and after hypothyroid treatment (p < 0.05). There was a significant decrease in INR fluctuation after treatment of hypothyroidism (p < 0.05). CONCLUSIONS: Since fluctuations of INR may be very harmful especially in the elderly population, carefully selected patients using warfarin with labile INR, can gain from concomitant therapy for subclinical hypothyroidism.