Accuracy of Serum Free Thyroxine Concentrations Determined by a New Veterinary Chemiluminscent Immunoassay in Euthyroid and Hyperthyroid Cats

Peterson M.E., Broome M.R. and Robertson J.E.

Conference Proceedings, (2011). European College of Veterinary Internal Medicine, Sevilla, Spain: p.1


Free thyroxine (fT4) is the unbound biologically active fraction of total thyroxine (TT4). The pituitary-thyroid axis functions to maintain fT4 within a certain range and fT4 may reflect thyroid function more accurately than TT4. Several methodologies are currently available to measure the concentration of fT4 in serum.

The purpose of this study was to establish the reference interval in cats for a new analog veterinary fT4 immunoassay, IMMULITE® 2000 Veterinary Free T4 (VfT4), Siemens Healthcare Diagnostics Products Ltd., Llanberis, Gwynedd, UK and to compare the accuracy of VfT4 to the other fT4 assays currently available for evaluation of cats with clinical signs of hyperthyroidism.

The VfT4 was compared to 2 fT4 radioimmunoassays by equilibrium dialysis, which included the Direct Free T4 by Dialysis (fT4EDIVD), IVD Technologies, Santa Ana, CA, USA and the Free T4 by Equilibrium Dialysis (fT4EDAN), Antech Diagnostics, Irvine, CA, USA. The VfT4 was also compared to a two-step direct fT4 immunoassay, GammacoatTM Free T4 (Two-Step) Radioimmunoassay (DfT4), Diasorin, Inc., Stillwater, MN, USA.

The study included 53 clinically healthy cats, which were confirmed to be euthyroid based upon history, physical examination, CBC, serum biochemistries, and TT4.  Free T4 concentrations were determined by each method. Technetium scans were performed on 11 of these cats; results were unremarkable.

Eighty-four cats with clinical signs of hyperthyroidism were examined. Complete histories, physical examinations, CBCs, serum biochemistries, TT4, fT4 by each method, and technetium scans were performed. Fifty-three cats had TT4 >  70nmol/L and 31 cats had TT4 of 30-70 nmol/L. Cats were classified as either euthyroid (n=6) or hyperthyroid (n=78) based upon results of the technetium scans.

The reference interval of VfT4 based on evaluation of the healthy euthyroid cats was determined to be 9.0 – 33.5 pmol/L. For cats with clinical signs of hyperthyroidism, the sensitivities of the immunoassays were: VfT4 (92%), fT4EDIVD (90%), fT4EDAN (95%), DfT4 (95%). The specificities were: VfT4 (100%), fT4EDIVD (100%), fT4EDAN (67%), DfT4 (100%). Overall accuracies of the fT4 tests were: VfT4 (93%), fT4EDIVD (91%), fT4EDAN (93%), DfT4 (95%).

Results of this study show that the accuracy of the new VfT4 immunoassay is comparable to other currently available fT4 assays for the diagnosis of hyperthyroidism in cats with clinical signs of this disease.