Comparison of serum-free thyroxine concentrations determined by standard equilibrium dialysis, modified equilibrium dialysis, and 5 radioimmunoassays in dogs

Measurement of serum-free thyroxine (fT₄) concentration provides a more accurate assessment of thyroid gland function than serum thyroxine (T₄) or 3,5,3′-triiodothyronine (T₃). Techniques for measuring serum fT₄ concentration include standard equilibrium dialysis (SED), radioimmunoassay (RIA), and a combination of both (modified equilibrium dialysis [MED]). This study compared results of serum fT₄ 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 fT₄ concentrations were comparable when determined by the SED and MED techniques, and mean serum fT₄ 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 fT₄ concentrations were identified among the 5 RIAs and among the RIAs and MED and SED. Serum fT₄ concentrations were consistently lower when fT ₄ was determined by the RIAs, compared with either equilibrium dialysis technique. Serum fT₄ 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 fT₄ concentrations in euthyroid dogs with concurrent illness. This study documented differences in test results among fT₄ assays, emphasizing the importance of maintaining consistency in the assay used to measure serum fT₄ concentrations in the clinical or research setting.