Serum free thyroxine concentration in healthy dogs, dogs with hypothyroidism, and euthyroid dogs with concurrent illness

Abstract

Summary Serum free thyroxine (fT4), thyroxine (T4), and 3,5,3'-triiodothyronine (T3) concentrations were determined in 62 healthy dogs, 51 dogs with hypothyroidism, and 59 euthyroid dogs with concurrent dermatopathy or concurrent illness for which hypothyroidism was a diagnostic consideration. Status of thyroid function was based on history, physical findings, results of thyrotropin response testing, requirement for thyroid hormone replacement therapy, and in 31 dogs, on results of histologic examination of a thyroid gland biopsy specimen. Serum fT4 concentration was determined, using a single-stage radioimmunoassay. Mean (±sd)serumfT4 concentration was significantly (P < 0.05) greater in healthy dogs vs dogs with hypothyroidism (0.51 ± 0.27 ng/dl vs 0.10 ± 0.07 ng/dl). Significant difference in mean serum fT4 concentration was not evident between dogs with hypothyroidism and euthyroid dogs with hyperadrenocorticism (0.16 ± 0.13 ng/dl) or peripheral neuropathy (0.19 ± 0.10 ng/dl). Mean serum fT4 concentration in all other groups of euthyroid dogs with concurrent illness was similar to values in healthy dogs and was significantly (P < 0.05) greater, compared with values in dogs with hypothyroidism. Similar results were found for mean serum T4 concentration. Comparison of serum fT4 vs T4 concentration revealed: sensitivity, 0.97 vs 0.98; specificity, 0.78 vs 0.73; predictive value for a positive test result, 0.79 vs 0.80; predictive value for a negative test result, 0.97 vs 0.97; and accuracy, 0.78 vs 0.86, respectively. Ten (17%) and 12 (20%) of 59 serum fT4 and T4 concentrations, respectively, were inappropriately low in euthyroid dogs with concurrent illness. Of euthyroid dogs with concurrent illness, those with hyperadrenocorticism, peripheral neuropathy, or idiopathic generalized megaesophagus had the lowest serum fT4 and T4 concentrations. Significant difference in mean serum T3 concentration was not detected among healthy dogs, dogs with hypothyroidism, or euthyroid dogs with concurrent illness. Measurement of serum fT4 concentration using the single-stage radioimmunoassay, did not provide additional information about thyroid gland function other than that gained by measurement of serum T4 concentration.