Comparison of Serum‐Free Thyroxine Concentrations Determined by Standard Equilibrium Dialysis, Modified Equilibrium Dialysis, and 5 Radioimmunoassays in Dogs

Measurement of serum‐free thyroxine (fT 4 ) concentration provides a more accurate assessment of thyroid gland function than serum thyroxine (T4) or 3,5,3'‐triiodothyronine (T 3 ). Techniques for measuring serum fT 4 concentration include standard equilibrium dialysis (SED), radioimmunoassay (RIA), and a combination of both (modified equilibrium dialysis [MED]). This study compared results of serum fT 4 measurements by means of SED, MED, and 5 RIAs in 30 healthy dogs, 10 dogs with hypothyroidism, and 31 euthyroid dogs with concurrent illness for which hypothyroidism was a diagnostic consideration. Serum fT 4 concentrations were comparable when determined by the SED and MED techniques, and mean serum fT 4 concentrations were significantly (P <.01) lower in dogs with hypothyroidism than in healthy dogs and euthyroid dogs with concurrent illness. Significant (P < .05) differences in fT 4 concentrations were identified among the 5 RIAs and among the RIAs and MED and SED. Serum fT 4 concentrations were consistently lower when fT 4 was determined by the RIAs, compared with either equilibrium dialysis technique. Serum fT 4 concentrations were significantly lower (P < .01) in dogs with hypothyroidism than in healthy dogs for all RIAs; were significantly lower (P < .05) in dogs with hypothyroidism than in euthyroid dogs with concurrent illness for 4 RIAs; and were significantly lower (P < .01) in euthyroid dogs with concurrent illness than in healthy dogs for 4 RIAs. RIAs had the highest number of low serum fT 4 concentrations in euthyroid dogs with concurrent illness. This study documented differences in test results among fT 4 assays, emphasizing the importance of maintaining consistency in the assay used to measure serum fT 4 concentrations in the clinical or research setting.