Effect of Non-Thyroidal Illness on Serum Concentrations of T4, Free T4 and Thyroid Stimulating Hormone in Cats

Davignon D., Lucy J., Randolph J.F., et al. (2015). ACVIM Forum. Indianapolis, Indiana: 558.


In cats, non-thyroidal illness (NTI) may result in decreased total thyroxine (T4) concentration, similar to the “euthyroid sick syndrome” in dogs. The pathogenesis of this phenomenon is not completely understood, but may involve suppression of thyroid stimulating hormone (TSH) and reduced thyroid hormone protein binding. Although high TSH and normal T4 concentrations have been reported in humans with chronic kidney, liver, or heart disease, low serum concentrations of T4 and TSH commonly develop with severe NTI in humans. Serum TSH concentrations have not been evaluated in cats with NTI, except for a small number of cats with mild chronic kidney disease in which TSH levels were normal. The objective of this study was to measure TSH in a larger series of cats suffering from a variety of mild-to-severe NTI to determine if the spectrum of TSH concentrations reported in humans also develops in cats. Serum T4, free T4 (FT4) by equilibrium dialysis, and TSH concentrations were measured in 120 clinically healthy cats and 46 cats presenting for evaluation of NTI. Median concentrations of T4, FT4, and TSH were compared using the Mann-Whitney U test with P ≤0.05 considered significant. In sick cats, illness was graded as mild (outpatient exam; n=31), moderate (short hospitalization for treatable condition; n=6), or severe (intensive hospitalization associated with substantial morbidity/mortality; n=9). Diseases in sick cats included cardiac (n=11), endocrine (n=4), hepatic (n=2), infectious (n=2), neoplastic (n=11), renal (n=5), respiratory (n=1), and other (n=10) disorders. Median serum concentrations of T4 (1.5 μg/dL vs. 2.0 μg/dL) and FT4 (26 pmol/L vs. 32 pmol/L) were significantly (P <0.05) lower in the 46 sick cats compared to the healthy cats, respectively. However, no significant difference in median TSH concentration was detected between ill and healthy cats (Figure 1). Compared to the 31 sick cats with mild disease, the 9 cats with severe illness had significantly (P <0.05) lower median serum concentrations of T4, FT4, and TSH (Figure 2). In one cat with mild neoplastic disease, high TSH concentration with normal T4 and FT4 was identified (Figures 1 & 2). In conclusion, our data demonstrate that median serum concentrations of T4 and FT4 are lower in sick compared to healthy cats, and these parameters may be inversely correlated with disease severity. Overall, TSH concentrations in sick cats were not significantly different from healthy cats. However, TSH levels in cats may decrease in severe illness, and may occasionally be high, similar to findings in humans with NTI.