Effects of phenobarbital treatment on serum thyroxine and thyroid-stimulating hormone concentrations in epileptic dogs

Cynthia L. Gaskill From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Shelley A. Burton From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Hans C. J. Gelens From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Sherri L. Ihle From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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James B. Miller From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Darcy H. Shaw From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Michael B. Brimacombe From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Alastair E. Cribb From the Laboratory of Pharmacogenetics, Department of Anatomy and Physiology (Gaskill, Cribb), and Departments of Pathology and Microbiology (Burton), Companion Animals (Gelens, Ihle, Miller, Shaw), and Health Management (Brimacombe), Atlantic Veterinary College, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A 4P3.

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Objective

To determine whether phenobarbital treatment of epileptic dogs alters serum thyroxine (T4) and thyroid-stimulating hormone (TSH) concentrations.

Design

Cross-sectional study.

Animals

78 epileptic dogs receiving phenobarbital (group 1) and 48 untreated epileptic dogs (group 2).

Procedure

Serum biochemical analyses, including T4 and TSH concentrations, were performed for all dogs. Additional in vitro analyses were performed on serum from healthy dogs to determine whether phenobarbital in serum interferes with T4 assays or alters free T, (fT4) concentrations.

Results

Mean serum T4 concentration was significantly lower, and mean serum TSH concentration significantly higher, in dogs in group 1, compared with those in group 2. Thirty-one (40%) dogs in group 1 had serum T4 concentrations less than the reference range, compared with 4 (8%) dogs in group 2. All dogs in group 2 with low serum T4 concentrations had recently had seizure activity. Five (7%) dogs in group 1, but none of the dogs in group 2, had serum TSH concentrations greater than the reference range. Associations were not detected between serum T4 concentration and TSH concentration, age, phenobarbital dosage, duration of treatment, serum phenobarbital concentration, or degree of seizure control. Signs of overt hypothyroidism were not evident in dogs with low T4 concentrations. Addition of phenobarbital in vitro to serum did not affect determination of T4 concentration and only minimally affected fT4 concentration.

Conclusions and Clinical Relevance

Clinicians should be aware of the potential for phenobarbital treatment to decrease serum T4 and increase TSH concentrations and should use caution when interpreting results of thyroid tests in dogs receiving phenobarbital. (J Am Vet Med Assoc 1999:215:489–496)

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Cover Journal of the American Veterinary Medical Association
Journal of the American Veterinary Medical Association
Issue:
15 Aug 1999
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