fa عمومى General پژوهشي Research <div class="rounded" style="background: rgb(238, 238, 238); border: 1px solid rgb(204, 204, 204); padding: 5px 10px; text-align: justify;"><span style="line-height:2;"><span style="font-family:Times New Roman;"><span style="font-size:14px;"><span style="color:#2980b9;"><b>Background</b>: </span>Warfarin anticoagulation in cardiology is strongly influenced by thyroid status, which alters vitamin K&ndash;dependent clotting factor turnover; hyperthyroidism increases warfarin sensitivity, whereas hypothyroidism causes relative resistance and requires higher doses. In addition, starting or up‑titrating levothyroxine can further enhance warfarin sensitivity, while antithyroid therapy with methimazole may blunt its anticoagulant effect and necessitate higher doses.<br> <span style="color:#2980b9;"><b>Methods</b>:</span> This retrospective case series was conducted in a cardiology clinic from 2023 to 2025 and included stable chronic warfarin users (>3 months within therapeutic INR) who initiated levothyroxine or methimazole and subsequently developed markedly supratherapeutic (INR>5.0) or subtherapeutic (INR<1.5) values. Data captured demographics, valve indications, thyroid function (thyroid-stimulating hormone and free thyroxine), INR trends, dose adjustments, and outcomes.<br> <span style="color:#2980b9;"><b>Results</b>:</span> The studied hypothyroid patients (three females, one male; ages 45-72 years) developed INR values ranging from 5.0 to 9.2 (14-28 days post-levothyroxine 50-150 mcg daily), with minor bleeding in two cases. Warfarin doses were reduced by 20-42%, normalizing INR within 2-4 weeks. Four hyperthyroid patients (three females, one male; ages 55-67 years) showed INR values of 1.0-1.3 (2-5 weeks post-methimazole 15-30 mg daily), without thromboses. Warfarin doses increased by 25-55%, with INR values stabilizing over 6 weeks.<br> <span style="color:#2980b9;"><b>Conclusion</b>:</span> Levothyroxine decreases warfarin dose requirements through accelerated clotting factor clearance; however, methimazole increases warfarin dose needs as euthyroidism develops. In valve replacement patients, weekly INR monitoring for 4&ndash;6 weeks after initiation of thyroid therapy is recommended, along with preemptive warfarin adjustments (20&ndash;40% reduction with levothyroxine; 25&ndash;50% increase with methimazole) and endocrinology consultation to minimize adverse events.</span></span></span></div> Anticoagulation, Cardiology, Case series, Drug interaction, INR, Levothyroxine, Methimazole, Pharmacodynamics, Thyroid disease, Warfarin 0 0 http://ajpr1.umsha.ac.ir/browse.php?a_code=A-10-2-10&slc_lang=fa&sid=1 Mahdis Sharifikia 100319475328460052 100319475328460052 No Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran Bahar Seifi-Zarei 100319475328460053 100319475328460053 No Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran Melika Shalileh 100319475328460054 100319475328460054 No Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran Masoumeh Sohrabi 100319475328460055 100319475328460055 No Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran Shabnam Rezaie 100319475328460056 100319475328460056 No Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran Maryam Mehrpooya 100319475328460057 100319475328460057 Yes Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran