Research Updates: Thyroid assays and NSAIDs: A bad combination?

In this prospective university study, 38 dogs receiving etodolac for either an orthopedic condition (21 dogs) or postoperative pain control (17 dogs) underwent thyroid hormone testing to determine the effect of the drug on laboratory values. None of the dogs had a thyroidal illness nor received medication known to alter thyroid function test results. The mean age of the dogs was 5.6 years, and the mean body weight was 69.3 lb (31.5 kg). Blood samples were obtained for measurements of total thyroxine (T4), free T4, and canine thyrotropin concentrations. The dosage of etodolac ranged from 10 to 13 mg/kg given once a day for 14 to 19 days.

The results after etodolac treatment indicated a significant decrease in mean serum T4, with 21% of the values falling below the reference range. There was a significant increase in canine thyrotropin concentrations after etodolac treatment. No significant changes occurred in free T4 concentrations, though 10% of the concentrations were below the reference range. The authors concluded that etodolac therapy should be discontinued before thyroid function testing in dogs. If this is not feasible, only free T4 should be measured.

COMMENTARY

Hypothyroidism is a common endocrinopathy, although it is frequently overdiagnosed because of alteration in thyroid function test results associated with concurrent drug therapy. Nonsteroidal anti-inflammatory drugs (NSAIDs) are frequently prescribed in veterinary patients, and these drugs competitively bind to plasma proteins with thyroxine. Theoretically, this should reduce T4, increase free T4, and decrease canine thyrotropin concentrations. Previously, a decrease in canine thyrotropin has been described in patients treated with another NSAID (carprofen), while another study found that etodolac had no effect on thyroid hormone concentrations. Although the results of this study are in conflict with these data and may relate to the timing of blood samples, it would seem prudent for clinicians to eliminate the use of NSAIDs when checking thyroid function in dogs.

Ness, T.A. et al.: Effect of dosing and sampling time on serum thyroxine, free thyroxine, and thyrotropin concentrations in dogs following multidose etodolac administration. Vet. Ther. 4 (4):340-349; 2003.