Free thyroxine concentrations by equilibrium dialysis and chemiluminescent immunoassays in 13 hypothyroid dogs positive for thyroglobulin antibody

Randolph JF, Lamb SV, Cheraskin JL, et al.

J Vet Intern Med 2015;29:877-881.

OBJECTIVE: To determine if concentrations of free thyroxine (FT4) measured by semi-automated chemiluminescent immunoassay (CLIA) correspond to FT4 determined by equilibrium dialysis (ED) in hypothyroid dogs positive for thyroglobulin antibody (TGA). ANIMALS: Thirteen TGA-positive dogs classified as hypothyroid based on subnormal FT4 concentrations by ED. METHODS: Qualitative assessment of canine TGA was performed using an enzyme-linked immunosorbent assay. Serum total thyroxine and total triiodothyronine concentrations were measured by radioimmunoassay. Serum FT4 concentration was determined by ED, and also by semi-automated CLIA for human FT4 (FT4h) and veterinary FT4 (FT4v). Canine thyroid stimulating hormone concentration was measured by semi-automated CLIA. RESULTS: Each dog’s comprehensive thyroid profile supported a diagnosis of hypothyroidism. For detection of hypothyroidism, sensitivities of CLIA for FT4h and FT4v were 62% (95% CI, 32-85%) and 75% (95% CI, 36-96%), respectively, compared to FT4 by ED. Five of 13 (38%) dogs had FT4h and 2 of 8 (25%) dogs had FT4v concentrations by CLIA that were increased or within the reference range. Percentage of false-negative test results for FT4 by CLIA compared to ED was significantly (P < .0001 for FT4h and P < .001for FT4v) higher than the hypothesized false-negative rate of 0%. CONCLUSIONS AND CLINICAL IMPORTANCE: Caution should be exercised in screening dogs for hypothyroidism using FT4 measured by CLIA alone. Some (25-38%) TGA-positive hypothyroid dogs had FT4 concentrations determined by CLIA that did not support a diagnosis of hypothyroidism.