Possible effect of thyroid function on anticoagulant response to unfractionated heparin

Many factors are known to influence the unfractionated heparin (UFH) dosage required to achieve therapeutic anticoagulation, including weight, sex, age, tobacco smoking, and diabetes mellitus. However, no interaction between thyroid function and UFH has been documented. An 83-year-old Caucasian woman had active hyperthyroidism that appeared to significantly increase her dosage requirements for UFH. The patient had atrial fibrillation secondary to the hyperthyroidism. She had no known factors that would increase UFH requirements. A UHF dosage of 1400 U/hour was needed to achieve a therapeutic level of anticoagulation (activated partial thromboplastin time 62 sec). After she was treated for hyperthyroidism, her atrial fibrillation resolved and UFH was discontinued. On day 9 of her hospital admission, atrial fibrillation resumed, and the patient's thyroid function began to normalize. Unfractionated heparin was restarted, and this time therapeutic anticoagulation was achieved with only 800 U/hour. This patient's experience suggests a previously undocumented and clinically significant relationship between active hyperthyroidism and altered dosage requirements for UFH. Recognition of this potential interaction may shorten the time required for patients with hyperthyroidism to achieve therapeutic anticoagulation, thereby preventing complications associated with subtherapeutic anticoagulation.