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 s everity is associate with abnormalities in baseline serum concentrations of total thyroxine (T-4), triiodothyronine (T-3), free T-4, or thyrotropin (t hyroid-stimulating hormone [TSH]) in dogs believed to be euthyroid. Design-Case-control study. Animals-223 dogs with confirmed nonthyroidal diseases and presumptive norma l 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 c linically normal dogs. Results-in dogs with nonthyroidal disease, median serum concentrations of t otal T-4, total T-3, and free T-4 were significantly lower than those in cl inically normal dogs. Median serum TSH concentration in sick dogs was signi ficantly greater than that of clinically normal dogs. When stratified by se verity of disease (ie, mild, moderate, and severe), dogs with severe diseas e had low serum concentrations of total T-4, total T-3, or free T-4 more co mmonly than did dogs with mild disease. In contrast, serum TSH concentratio ns were more likely to remain within the reference range regardless of seve rity of disease. Conclusions and Clinical Relevance-Results indicate that serum total T-4, f ree T-4, and total T-3 concentrations may be low (ie, in the hypothyroid ra nge) in dogs with moderate to severe nonthyroidal disease. Serum TSH concen trations are more likely to remain within the reference range in sick dogs.