Free Thyroxine Concentrations by Equilibrium Dialysis and Chemiluminescent Immunoassays in 13 Hypothyroid Dogs Positive for Thyroglobulin Antibody

Abstract

ObjectiveTo 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).AnimalsThirteen TGA‐positive dogs classified as hypothyroid based on subnormal FT4 concentrations by ED.MethodsQualitative 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.ResultsEach 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 ImportanceCaution 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.

Effects of Toceranib Phosphate on the Hypothalamic‐Pituitary‐Thyroid Axis in Tumor‐Bearing Dogs

“…Samples then were shipped in batches to Cornell University for the same analysis at the AHDC. Procedures used in thyroid testing of dogs by the AHDC have been described previously . Before May 1, 2014, the AHDC used a 2‐step radioimmunoassay to quantify FT4.…”

“…Procedures used in thyroid testing of dogs by the AHDC have been described previously. 22 Before May 1, 2014, the AHDC used a 2-step radioimmunoassay to quantify FT4. When commercial production of that kit ceased, the Immulite b assay was used.…”

Serum thyroxine and thyroid‐stimulating hormone concentration in hyperthyroid cats that develop azotaemia after radioiodine therapy

“…, Peterson , Randolph et al . ), with consequent variable diagnostic test performance, especially in cats with non‐thyroidal illness (Mooney et al . , Peterson et al .…”

Correlation of thyroid hormone measurements with thyroid stimulating hormone stimulation test results in radioiodine‐treated cats

“…Of the 21 cats whose age was known, the median age was 10 years old. [7][8][9][10][11][12][13][14] The majority of control cats included in the study were Domestic short-or long-haired cats (n = 31) and two purebred cats were Lavaltrie, Québec J5T 3S5, Canada) (n = 1), insulin (Lantus ® Sanofi 2905 Place Louis-R.-Renaud, Laval, QC H7V 0A3) (n = 1).…”

“…7 The diagnosis of iatrogenic hypothyroidism can be challenging due to the effect of concurrent illness on serum free thyroxine (fT4) and serum total thyroxine (tT4) concentrations, 8,9 the poor sensitivity and specificity of commercially available thyroid stimulating hormone (TSH) assays, none of which are specific to cats, 10 and the variability in performance of various fT4 assay methodologies. [11][12][13] Few studies have looked at the sensitivity and specificity of the available thyroid hormone analyses for the diagnosis of iatrogenic hypothyroidism in cats. 6,13 Traditionally tT4 has been used alone in cats to diagnose thyroid disease; however, tT4 has poor sensitivity and specificity for the diagnosis of iatrogenic hypothyroidism in this species, 6 especially when laboratory reference intervals (RIs) are used.…”