Endotoxin-induced nonthyroidal illness in dogs

Objective - To determine the effects of endotoxin administration on thyroid function test results and serum tumor necrosis factor-α (TNF-α) activity in healthy dogs. Animal - 6 healthy adult male dogs. Procedures - Serum concentrations of thyroxine (T₄), 3,5,3′-triiodothyronine (T₃), 3,3′5′-triiodothyronine (rT₃), free T₄ (fT₄), and endogenous canine thyroid stimulating hormone (TSH), and TNF-α activity were measured before (day-1; baseline), during (days 0 to 3), and after (days 4 to 24) IV administration of endotoxin every 12 hours for 84 hours. Results - Compared with baseline values, serum T₃ concentration decreased significantly, whereas rT₃ concentration increased significantly 8 hours after initial endotoxin administration. Serum T₄ concentration decreased significantly at 8 and 12 hours after initiating endotoxin administration. Serum T₄ concentration returned to reference range limits, then decreased significantly on days 6 to 12 and 16 to 20. Serum fT₄ concentration increased significantly at 12, 24, and 48 hours after cessation of endotoxin treatment, compared with baseline values. Serum rT₃ concentration returned to reference range, then decreased significantly days 5 and 7 after stopping endotoxin treatment. Serum TNF-α activity was significantly increased only 4 hours after initial endotoxin treatment, compared with baseline activity. Conclusions and Clinical Relevance - Endotoxin administration modeled alterations in thyroid function test results found in dogs with spontaneous nonthyroidal illness syndrome. A decrease in serum T₄ and T₃ concentrations and increase in serum rT₃ concentration indicate impaired secretion and metabolism of thyroid hormones. The persistent decrease in serum T₄ concentration indicates that caution should be used in interpreting serum T₄ concentrations after resolution of an illness in dogs.