Assessment of Thyroid Function in Dogs with Low Plasma Thyroxine Concentration

Abstract

Background : Differentiation between hypothyroidism and nonthyroidal illness in dogs poses specific problems, because plasma total thyroxine (TT 4 ) concentrations are often low in nonthyroidal illness, and plasma thyroid stimulating hormone (TSH) concentrations are frequently not high in primary hypothyroidism. Hypothesis : The serum concentrations of the common basal biochemical variables (TT 4 , freeT 4 [fT 4 ], and TSH) overlap between dogs with hypothyroidism and dogs with nonthyroidal illness, but, with stimulation tests and quantitative measurement of thyroidal 99m TcO 4 ‐ uptake, differentiation will be possible. Animals : In 30 dogs with low plasma TT 4 concentration, the final diagnosis was based upon histopathologic examination of thyroid tissue obtained by biopsy. Fourteen dogs had primary hypothyroidism, and 13 dogs had nonthyroidal illness. Two dogs had secondary hypothyroidism, and 1 dog had metastatic thyroid cancer. Methods : The diagnostic value was assessed for (1) plasma concentrations of TT 4 , fT 4 , and TSH; (2) TSH‐stimulation test; (3) plasma TSH concentration after stimulation with TSH‐releasing hormone (TRH); (4) occurrence of thyroglobulin antibodies (TgAbs); and (5) thyroidal 99m TcO 4 ‐ uptake. Results : Plasma concentrations of TT 4 , fT 4 , TSH, and the hormone pairs TT 4 /TSH and fT 4 /TSH overlapped in the 2 groups, whereas, with TgAbs, there was 1 false‐negative result. Results of the TSH‐ and TRH‐stimulation tests did not meet earlier established diagnostic criteria, overlapped, or both. With a quantitative measurement of thyroidal 99m TcO 4 ‐ uptake, there was no overlap between dogs with primary hypothyroidism and dogs with nonthyroidal illness. Conclusions and Clinical Importance : The results of this study confirm earlier observations that, in dogs, accurate biochemical diagnosis of primary hypothyroidism poses specific problems. Previous studies, in which the TSH‐stimulation test was used as the “gold standard” for the diagnosis of hypothyroidism may have suffered from misclassification. Quantitative measurement of thyroidal 99m TcO ‐ uptake has the highest discriminatory power with regard to the differentiation between primary hypothyroidism and nonthyroidal illness.