Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in dogs with nonthyroidal disease

Objective-To determine whether nonthyroidal disease of various causes and severity is associate with abnormalities in baseline serum concentrations of total thyroxine (T-4), triiodothyronine (T-3), free T-4, or thyrotropin (thyroid-stimulating hormone [TSH]) in dogs believed to be euthyroid. Design-Case-control study. Animals-223 dogs with confirmed nonthyroidal diseases and presumptive normal thyroid function, and 150 clinically normal dogs. Procedure-Serum total T-4, total T-3, free T-4, and TSH concentrations were measured in dogs with confirmed nonthyroidal disease, Reference ranges for hormone concentrations were established on the basis of results from 150 clinically normal dogs. Results-in dogs with nonthyroidal disease, median serum concentrations of total T-4, total T-3, and free T-4 were significantly lower than those in clinically normal dogs. Median serum TSH concentration in sick dogs was significantly greater than that of clinically normal dogs. When stratified by severity of disease (ie, mild, moderate, and severe), dogs with severe disease had low serum concentrations of total T-4, total T-3, or free T-4 more commonly than did dogs with mild disease. In contrast, serum TSH concentrations were more likely to remain within the reference range regardless of severity of disease. Conclusions and Clinical Relevance-Results indicate that serum total T-4, free T-4, and total T-3 concentrations may be low (ie, in the hypothyroid range) in dogs with moderate to severe nonthyroidal disease. Serum TSH concentrations are more likely to remain within the reference range in sick dogs.