Serum thyroxine and thyrotropin concentrations decrease with severity of nonthyroidal illness in cats and predict 30‐day survival outcome

BACKGROUND: In cats, nonthyroidal illness affects serum thyroid hormone concentrations. Serum thyroxine (T(4)) and triiodothyronine (T(3)) concentrations commonly decrease, whereas free T(4) (fT(4)) concentrations vary unpredictably. Limited information exists regarding effects on serum thyrotropin (thyroid‐stimulating hormone [TSH]) concentrations in cats with nonthyroidal illness syndrome (NTIS). OBJECTIVES: To characterize alterations in thyroid function that develop in cats with NTIS and to correlate these alterations with severity and outcome of the nonthyroidal illness. ANIMALS: Two hundred and twenty‐two cats with NTIS and 380 clinically normal cats of similar age and sex. METHODS: Prospective, cross‐sectional study. All cats had serum T(4), T(3), free T(4), and TSH concentrations measured. Cats were grouped based on illness severity and 30‐day survival. RESULTS: Cats with NTIS had lower serum T(4) and T(3) concentrations than did normal cats (P < .001). Serum fT(4) and TSH concentrations did not differ between groups. Serum T(4), T(3), and fT(4) concentrations progressively decreased with increasing disease severity (P < .001). The 56 cats that died had lower T(4), T(3), and TSH concentrations than did the 166 survivors, with no difference in fT(4) concentration. Multivariable logistic regression modeling indicated that serum T(4) and TSH concentrations both predicted survival (P < .02). CONCLUSIONS AND CLINICAL IMPORTANCE: Cats with NTIS commonly develop low serum T(4), T(3), and TSH concentrations, the prevalence and extent of which increases with disease severity. Clinicians should consider evaluating thyroid function in cats with severe NTIS, because doing so could help determine probability of successful treatment responses before investing considerable time, effort, and finances in addressing the underlying disease.