Every Cat Logo

Determining the severity of nonthyroidal illness in cats

Peterson M, Davignon D, Shaw N, Dougherty E, Rishniw M, Randolph J. Serum thyroxine and thyrotropin concentrations decrease with severity of nonthyroidal illness in cats and predict 30-day survival outcome. J Vet Intern Med. 2020.

Thyroid hormone testing is routinely performed in cats to screen for and diagnose hyperthyroidism, a common condition of thyroid hormone overproduction. However, low thyroid hormone levels are often seen in systemic illness in many species, a syndrome referred to as “nonthyroidal illness syndrome” (NTIS), previously called “sick euthyroid syndrome”. In humans and dogs, this is usually characterized by a low serum total T4 and T3, normal free T4, and normal to (uncommonly) elevated TSH. In both of these species, a low T3, TT4, or TSH are predictive of mortality. In cats, a low TT4 has been shown to be associated with death in cases of panleukopenia, however T3 and TSH have not been evaluated. The purpose of this study was to determine the effect of non thyroidal illness on TT4, fT4, T3, and TSH, to determine if class or severity of illness had different effects on these values, and finally to determine if abnormalities in these hormones could predict outcome and survival.

The study was designed as a prospective cross-sectional study including clinically normal cats and cats with non-thyroidal illness. Cats with a history of hyperthyroidism were excluded. Three hundred eighty normal cats were recruited as controls, and to establish reference intervals for thyroid hormones. Cats were normal based on PE, serum biochemistry, CBC, and urinalysis. 222 cats with non-thyroidal illness were also recruited. Diagnosis of non-thyroidal illness was based on blood and urine tests, imaging, and pathology findings. No cats had received medications in the last two weeks known to affect thyroid function.

Cats with non-thyroidal illness (NTIS) were classified as mild, moderate (brief hospitalization recommended), or severe (requiring intensive care). Cats were also divided into 10 groups based on disease category (ie renal, GI, neurologic, etc).

The 380 euthyroid cats ranged from 1-18 years (median 10 years), with an approximately equal distribution of male and female. NTIS cats ranged from 1-19 years (median 11 years). 37% had mild disease, moderate in 32%, and severe in 31%. 23% had renal/urinary disease, 20% neoplastic, 13% GI, 10% hepatic, 9% endocrine, 8% infectious, 7% cardiac, 5% respiratory, 4% neurologic, and 1% dermatologic.

Cats with NTIS had significantly lower TT4 and T3 concentrations than healthy cats. fT4 did not differ between groups, however cats with NTIS had a higher percentage of elevated fT4 than healthy cats. TSH did not differ between groups. When stratified into mild, moderate, and severe illness, there was a progressive decrease in TT4, fT4, and T3 concentrations as illness worsened. TSH did not differ between groups, however mild and moderate pooled had a higher TSH than severe.

Cats who died or were euthanized had a lower TT4, T3, and TSH than cats who survived, however fT4 did not differ. TT4 had the greatest prognostic effect on outcome, however a predictive curve could be produced using TT4, fT4, T3 and TSH with a 0.789 AUCROC. For each 5mol/L decrease in TT4, there is a 56% increase in the odds of dying in the next 30 days.

The authors conclude that TT4, T3, and TSH decrease in non-thyroidal illness in cats, with a less reliable effect on fT4, and that these may be used as predictors of mortality.

Limitations to this study were present. Scintigraphy was not used to rule out non thyroidal illness in all cats. The presence of multiple disease may have influenced categorization of disease. TSH testing in cats also had notoriously poor analytical sensitivity, which may have influenced measurement. (MRK)

See also:

Petini M, Drigo M, Zoia A. Prognostic value of systemic inflammatory response syndrome and serum concentrations of acute phase proteins, cholesterol, and total thyroxine in cats with panleukopenia. J Vet Intern Med. 2020; 34: 719‐ 724.