Mooney C.T., Little C.J. and Macrae A.W.
OBJECTIVE–To determine circulating concentrations and fate of total and free thyroxine (T4) in cats with various illnesses not associated with the thyroid glands (nonthyroidal 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.