Nonthyroidal Illness Syndrome in Adult Horses

Background This study was performed to determine whether sick horses have thyroid hormone ( TH ) alterations similar to those observed in nonthyroidal illness syndrome in other species. Hypothesis Horses suffering from systemic diseases have decreased TH s and inappropriately low thyroid‐stimulating hormone ( TSH ). Animals Seventy‐one clinically normal horses; 380 hospitalized horses. Methods Total thyroxine ( TT 4), free thyroxine by equilibrium dialysis ( fT 4D), total triiodothyronine ( TT 3), free triiodothyronine ( fT 3), and TSH were measured in normal and hospitalized horses. Disease severity was categorized as mild, moderate, or severe by both subjective and objective criteria. Results Negative correlations existed between all TH s, except TSH , and objective illness severity scores. These scores also increased with each subjective disease severity category. TT 3 and fT 3 were decreased with mild disease. TT 3 progressively decreased more with moderate and severe disease. TT 4 and fT 4D remained normal with mild disease, but decreased progressively with disease severity. TSH increased with mild disease, but remained normal with moderate or severe disease. Horses that died or were euthanized had lower concentrations of all TH s, except TSH , when compared with those that lived. In horses that received >3 doses of NSAIDs, corticosteroids, or heparin compared to 0–3 doses, TT 3 and TT 4 were decreased, whereas fT 4D and TSH remained normal. There were minimal TH changes in horses that were not eating. Conclusions and Clinical Importance Thyroid hormones decrease in horses with systemic disease. TT 3 decreases first, followed by TT 4 and fT 4D. TSH fails to increase proportionally to the changes in TH s, indicating hypothalamic–pituitary axis dysregulation. NSAIDs, corticosteroids, heparin, and fasting have less effect on TH s compared with disease severity.