Volume 5, Issue 1 (Avicenna Journal of Pharmaceutical Research 2024)
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Sharifikia M, Seifi-Zarei B, Shalileh M, Sohrabi M, Rezaie S, Mehrpooya M. Warfarin-Thyroid Drug Interactions in Cardiology Practice: A Case Series. Avicenna J Pharma Res 2024; 5 (1)
URL: http://ajpr1.umsha.ac.ir/article-1-30-en.html
URL: http://ajpr1.umsha.ac.ir/article-1-30-en.html
Mahdis Sharifikia1 
, Bahar Seifi-Zarei1 , Melika Shalileh1 , Masoumeh Sohrabi1 , Shabnam Rezaie1 , Maryam Mehrpooya *1 
, Bahar Seifi-Zarei1 , Melika Shalileh1 , Masoumeh Sohrabi1 , Shabnam Rezaie1 , Maryam Mehrpooya *1
1- Department of Clinical Pharmacy, School of Pharmacy, Hamadan University of Medical Sciences, Hamadan, Iran
Abstract: (61 Views)
Background: Warfarin anticoagulation in cardiology is strongly influenced by thyroid status, which alters vitamin K–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.
Methods: 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.
Results: 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.
Conclusion: 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 4–6 weeks after initiation of thyroid therapy is recommended, along with preemptive warfarin adjustments (20–40% reduction with levothyroxine; 25–50% increase with methimazole) and endocrinology consultation to minimize adverse events.
Methods: 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.
Results: 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.
Conclusion: 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 4–6 weeks after initiation of thyroid therapy is recommended, along with preemptive warfarin adjustments (20–40% reduction with levothyroxine; 25–50% increase with methimazole) and endocrinology consultation to minimize adverse events.
Keywords: Anticoagulation, Cardiology, Case series, Drug interaction, INR, Levothyroxine, Methimazole, Pharmacodynamics, Thyroid disease, Warfarin
Type of Study: Research |
Subject:
General
Received: 2026/08/25 | Accepted: 2024/06/30 | Published: 2024/06/30
Received: 2026/08/25 | Accepted: 2024/06/30 | Published: 2024/06/30
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