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Background: Nonthyroidal illness syndrome (NTIS) can result in thyroid function test alterations that mimic hypothyroidism. The duration of NTIS‐induced changes in dogs is not well‐described. Objectives: Document alterations in thyroid function tests during NTIS and recovery, and the time necessary for their resolution. Animals: From 103 dogs sampled, 25 euthyroid dogs with acute, resolvable illness having a low serum total thyroxine (TT4) concentration on admission were analyzed. Methods: Prospective observational study. Serum TT4 concentration was measured in 103 dogs within 4 hours of admission. If below the reference interval (RI), subsequent serum samples were obtained every 24 hours from admission until discharge (acute phase) and at 2 weeks and 4 weeks after discharge (recovery phase). Serum samples were submitted for batch measurement of serum TT4, free thyroxine (fT4), total 3,5,3′‐triiodothyronine (TT3), and thyroid‐stimulating hormone (TSH) concentrations. Results: In the cohort of dogs analyzed, serum TT4, TT3, and fT4 concentrations were below the RI in 100%, 80%, and 16% at admission; 20%, 80%, and 0% at discharge; 4%, 8%, and 0% at 2 weeks; and 0%, 0%, and 0% at 4 weeks, respectively. Serum TSH concentration was within the RI in 100% at admission and discharge, and above the RI in 4% and 12% at 2 weeks and 4 weeks, respectively. Conclusions and Clinical Importance: Naturally occurring NTIS in dogs induces alterations in thyroid function tests during acute illness and recovery. Measurement of serum TT4 concentration 2 to 4 weeks after discharge or serum fT4 concentration by ED during illness is recommended for accurate assessment of thyroid function in acutely ill dogs. [ABSTRACT FROM AUTHOR]

Erroneous thyroid function test results can occur because of drugs that alter thyroid hormone physiology in one or more aspects, including synthesis, secretion, distribution, and metabolism. Research since publication of the last review in the Journal of Veterinary Internal Medicine (JVIM) 20 years ago has evaluated the effects of amiodarone, zonisamide, inhalant anesthetics, clomipramine, trilostane, and toceranib on thyroid function tests in the dog. In addition, recent work on the effects of glucocorticoids, sulfonamides, phenobarbital, and nonsteroidal anti‐inflammatory drugs will be reviewed. Awareness of these effects is necessary to avoid misdiagnosis of hypothyroidism and unnecessary treatment. [ABSTRACT FROM AUTHOR]