Syme HM, Elliott J, Dixon RM: Evaluation of free thyroxine measurement for the diagnosis of hyperthyroidism in cats With chronic renal failure. Proceedings of the 12th annual congress of the European College of Veterinary Internal Medicine-Companion animals, 2002.

Diagnosis of hyperthyroidism in cats with chronic renal failure (CRF) can be problematic, as many cats will have normal total thyroxine (TT4) due to the sick euthyroid syndrome. Free thyroxine (fT4) may be more sensitive in detecting cats with hyperthyroidism in the face of concurrent illness, however previous studies have identified frequent false positive test results. The purpose of this study was to evaluate fT4 as a test for hyperthyroidism in cats with CRF.

TT4 and fT4 (by equilibrium dialysis) were measured in cats with hyperthyroidism and CRF (HTh/CRF group, n=16) and in cats with CRF only (n=16). Cats were included in the HTh/CRF group if TT4 values were in the normal or 'questionable' range (<60 nmol/l) but were subsequently confirmed as hyperthyroid either by an abnormal TT4 measurement (>60 nmol/l, n=12) or an abnormal T3 suppression test (n=4). Cats were included in the CRF only group if they had no historical, physical or biochemical findings suggestive of hyperthyroidism, and remained clinically euthyroid for 6 months.

In the CRF only group TT4 and fT4 were 18.2 ± 8.3 nmol/l and 39.7 ± 30.8 pmol/l, respectively. Using the given laboratory reference range (10-40 pmol/l) the sensitivity and specificity of fT4 for diagnosis of hyperthyroidism were 100% and 69%. Using reference ranges generated from the CRF only group, sensitivities and specificities for fT4 (>100 pmol/l, excluding one outlier) were 50% and 94% and for TT4 (>35 nmol/l) were 69% and 94%. The area under receiver operating curves for fT4 (0.89, 95% CI 0.75-1.0) and TT4 (0.96, 95% CI 0.88-1.0) were not significantly different. Free T4 measurement confers no diagnostic advantage over TT4 if an identical method is used to generate the reference range for both tests.