Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats

Abstract

Objective - To determine circulating concentrations and fate of total and free thyroxine (T4) in cats with various illnesses not associated with the thyroid glands (non-thyroidal illnesses). Design - Prospective study. Animals - 98 cats with nonthyroidal illness and 50 clinically normal cats. Procedure - Serum total T4 concentrations were measured by radioimmunoassay, and serum free T4 concentrations were measured by direct equilibrium dialysis. Free T4 fraction was calculated from these 2 values. Results - Serum total T4 concentrations were significantly (P < 0.001) lower in sick cats (mean ± SD, 17.18 ± 8.14 nmol/L), compared with healthy cats (mean ± SD, 26.00 ± 7.62 nmol/L). Serum total T4 concentrations were inversely correlated with mortality. Differences in serum free T4 concentrations in sick cats (mean ± SD, 27.70 ± 13.53 pmol/L), compared with healthy cats (mean ± SD, 24.79 ± 8.33 pmol/L), were not significant. A few sick cats had serum free T4 concentrations greater than the reference range. Calculated free T4 fraction was significantly (P < 0.001) greater in sick cats (mean ± SD, 0.24 ± 0.30%), compared with healthy cats (mean ± SD, 0.10 ± 0.06%). Clinical Implications - Euthyroidism is maintained in sick cats, despite low serum total T4 concentrations. Measurement of serum total T4 concentrations is a valuable prognostic indicator. Serum free T4 concentrations should be used cautiously as a sole diagnostic criterion for confirmation of hyperthyroidism.