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Letters |1 January 1996

Increase in Serum Free Thyroxine Levels Related to Intravenous Heparin Treatment

Rita Jain, MD; Harry L. Uy, MD

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Author, Article, and Disclosure Information
Copyright ©2004 by the American College of Physicians
2004
  • University of Texas Health Science Center at San Antonio; San Antonio, TX 78284
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TO THE EDITOR:
Heparin has been shown to affect thyroid function test results [1, 2]. Because heparin is now commonly used to treat patients with unstable angina or atrial arrhythmias, physicians should be aware of this interaction. We describe a patient with a heparin-induced increase in the serum free thyroxine level as measured by direct equilibrium dialysis and a one-step free thyroxine immunoassay.
A 62-year-old woman with diabetes mellitus, hypertriglyceridemia, and coronary artery disease presented with unstable angina. On physical examination she appeared to be euthyroid, with no palpable thyroid nodules or goiter. She began receiving intravenous heparin, and the serum free thyroxine level measured during heparin treatment using an in-house assay (Ciba Corning, Medfield, Massachusetts) was 21.9 pmol/L. Repeat measurement of free thyroxine in the same serum sample showed a level of 32.2 pmol/L; the serum thyroid-stimulating hormone level was normal at 4.2 mU/L (normal range, 0.35 to 5.5 mU/L). To determine the cause of increased free thyroxine levels, total thyroxine, free thyroxine as measured by equilibrium dialysis, and free (nonesterified) fatty acid levels were determined in available blood samples that were collected into heparin-free tubes at hospital admission before heparin treatment, during heparin treatment, and 3 days after heparin infusion. As shown in Table 1, total thyroxine levels were not affected; the serum free thyroxine level increased only while the patient was receiving heparin and returned to baseline after therapy was discontinued.
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Jain R, Uy HL. Increase in Serum Free Thyroxine Levels Related to Intravenous Heparin Treatment. Ann Intern Med. ;124:74–75. doi: 10.7326/0003-4819-124-1_Part_1-199601010-00022

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Published: Ann Intern Med. 1996;124(1_Part_1):74-75.

DOI: 10.7326/0003-4819-124-1_Part_1-199601010-00022

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Print ISSN: 0003-4819 | Online ISSN: 1539-3704

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