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

Abstract

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 fT4 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.

Clinical Evaluation of a Novel Liquid Formulation of L‐Thyroxine for Once Daily Treatment of Dogs with Hypothyroidism

“…The inclusion criteria selected in the present study were stringent enough to confirm the presence of primary hypothyroidism in all cases. Furthermore, the method for evaluation of fT4 concentration, after equilibrium dialysis, is the gold standard for this variable and provides reliable values 13 . Finally, the use of TgAA test in dogs with low serum TSH concentrations was in accordance with recent scientific findings 5 …”

Obesity increases free thyroxine proportionally to nonesterified fatty acid concentrations in adult neutered female cats

“…In studies of healthy euthyroid dogs, a good correlation between dialysis and analog FT 4 immunoassays has been observed (Schachter et al 2004, Martin et al 2006. However, this correlation tends to degrade when sick animals are evaluated suggesting that these assays may be incapable of discerning the effects of weak circulating weak inhibitors of serum TH binding (Schachter et al 2004).…”

Hypothyroidism

“…In MED assays, a short dialysis step is used to separate free from protein-bound T 4 followed by radioimmunoassay for fT 4 . It is important to use a fT 4 assay that has been demonstrated to have adequate diagnostic performance in dogs, because some human analog assays for fT 4 have accuracy that is no better than measurement of total T 4 in dogs (Schachter, 2004), whereas others compare favorably to the MED methods (Table 3-8) (Scott-Moncrieff et al, 2011). MED techniques are regarded as the most accurate commercially-available technique for determining serum fT 4 concentrations in dogs.…”