Conference Proceedings, (2002). 12th ECVIM-CA/ESVIM Congress 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 sick euthyroid syndrome. Free thyroxine (ff4) 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 IT4 as a test for hyperthyroidism in cats with CRF. TT4 and IT4 (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 TI4 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 ff4 were 18.2 ± 8.3 nmolll and 39.7 ± 30.8 pmol/I, respectively. Using the given laboratory reference range (10-40 pmol/I) the sensitivity and specificity of IT4 for diagnosis of hyperthyroidism were 100% and 69%. Using reference ranges generated from the CRF only group, sensitivities and specificities for ff4 (>100 pmol/l, excluding one outlier) were 50% and 94% and for TT4 (>35 nmol/I) were 69% and 94%. The area under receiver operating curves for ff4 (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.