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 2 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

Adenohypophyseal Function in Dogs with Primary Hypothyroidism and Nonthyroidal Illness

“…PH was excluded by results of thyroid uptake of 99m TcO 4 À and histopathologic examination of thyroid biopsy samples. 8 All 13 dogs had normal thyroid tissue with the usual variation in follicle size. The epithelial cells were cubical, with some resorption vacuoles in the adjacent colloid.…”

“…Nine dogs were male (3 castrated) and 5 dogs were female (1 spayed , and extensive loss of thyroid tissue on histopathologic examination of biopsy samples of the thyroid gland. 8 In 11 dogs, generalized infiltration of lymphocytes and occasionally plasma cells was observed in the biopsy specimens. The remaining 3 dogs had complete atrophy of thyroid tissue without inflammatory infiltration.…”

“…8 Briefly, the 14 hypothyroid dogs were of various breeds and their ages ranged from 2 to 10 years (median, 5 years). Nine dogs were male (3 castrated) and 5 dogs were female (1 spayed , and extensive loss of thyroid tissue on histopathologic examination of biopsy samples of the thyroid gland.…”

“…[5][6][7] However, the gold standard status of the TSH stimulation test has been questioned. 8 Techniques not involving a biochemical assessment of the pituitary-thyroid axis, such as radionuclide imaging or uptake of 99m TcO 4 À and highresolution ultrasonography, seem to be the most reliable methods for diagnosing PH in dogs. 8,9 Several assay modifications have been suggested to improve the low diagnostic value of TSH measurements.…”

“…8 Techniques not involving a biochemical assessment of the pituitary-thyroid axis, such as radionuclide imaging or uptake of 99m TcO 4 À and highresolution ultrasonography, seem to be the most reliable methods for diagnosing PH in dogs. 8,9 Several assay modifications have been suggested to improve the low diagnostic value of TSH measurements. 4 However, there is now evidence that it is not necessarily the assay but changes in pituitary function that are responsible for the low TSH concentrations found in some dogs with PH.…”