Thyroid function tests in the dog. Recent concepts

Laboratory indicators of hypothyroidism and TgAA-positivity in the Eurasian dog breed

BACKGROUND

Hereditary hypothyroidism represents a concern for dog breeders; thus, surveillance programs have been established for several dog breeds.

METHODS

Thyroid profiles (total thyroxine (TT4), thyrotropin (thyroid stimulating hormone (TSH)), and thyroglobulin autoantibodies (TgAA)) collected as part of a breed surveillance program in Eurasians (2009-2017) were retrospectively analyzed. The study included data from 1,501 Eurasians from a German breeding club. Classification was exclusively based on laboratory data. Hypothyroidism was defined as a combined decrease in TT4 and increase in TSH in serum and was classified as TgAA-positive and TgAA-negative hypothyroidism. Thyroglobulin autoantibodies (TgAA) independent of the concentrations of TT4 and TSH were determined. The overall prevalence of hypothyroidism, TgAA-positive hypothyroidism, TgAA-negative hypothyroidism and TgAA-positivity was assessed when the dogs entered the program. Follow-up laboratory data was available for 324 dogs without hypothyroidism on initial examination.

RESULTS

The initial screening was performed at a median age of 18 months (interquartile range (IQR): 15-29). The overall prevalence of hypothyroidism was 3.9% (n = 58; 95% CI: 2.9-4.8%) and the prevalence of a positive TgAA status was 7.9% (n = 118; 95% CI: 6.6-9.3%). The prevalence of TgAA-positive and TgAA-negative hypothyroidism was 1.7% (n = 26; 95% CI: 1.1-2.4%) and 2.1% (n = 32; 95% CI: 1.4-2.9%), respectively. 22.0% of dogs with positive TgAA status (26/118) were already hypothyroid on initial examination. Overall, 42.5% (17/40) of TgAA-positive dogs on initial examination developed hypothyroidism on follow-up.

CONCLUSION

The results of this study demonstrate that the Eurasian dog breed exhibits a relevant risk for hypothyroidism and presence of TgAA. The predictive value of TgAA for hypothyroidism or developing hypothyroidism was high in this breed. Further investigations with longitudinal studies in individual dogs are warranted.

Testing for hypothyroidism in dogs

Hypothyroidism is the most common endocrinopathy in the dog. Rather than being a comprehensive review of all possible thyroid function tests, the focus in this article is on the logical progression of test choice, highlighting total thyroxine, free thyroxine, triiodothyronine, thyrotropin (TSH), and antithyroid antibodies. This article includes extensive discussion of the current status of the canine TSH assay and the potential for improving this assay.

Influence of drugs on thyroid function in dogs

Several drugs can affect thyroid function test results in humans and eventually lead to an erroneous evaluation of thyroid function. These medications can alter the synthesis, secretion, transport, or metabolism of thyroid hormones. Some drugs also directly inhibit the hypothalamic-pituitary-thyroid axis. The effects of drugs on thyroid function in dogs have long been underestimated and have most likely contributed to the overdiagnosis of hypothyroidism in this species. This manuscript 1st reviews pertinent thyroid physiology followed by an overview of drugs for which the effects on canine thyroid function have been studied. The effects of glucocorticoids, propranolol, sulfonamides, phenobarbital, potassium bromide, and nonsteroidal anti-inflammatory agents (NSAIDs) on canine thyroid function are summarized here. Knowledge of the potential effect of these medications on thyroid function should contribute to a more reliable interpretation of thyroid function test results in dogs.

Endocrinopathies. Thyroid and adrenal disorders

This article focuses on common adrenal and thyroid diseases in the geriatric patient consisting of hypothyroidism in the dog, hyperthyroidism in the cat, and hyperadrenocorticism in the dog to include clinical signs, diagnosis, and management. A brief section on hyperadrenocorticism in the cat, thyroid tumors in the dog, and pheochromocytoma in the dog and cat are also included.

Serum thyroid hormone concentrations in New Zealand horses

Total thyroxine and total tri-iodothyronine concentrations were measured in the sera from 125 horses of mixed age, breed and sex, and varied clinical histories. While low serum thyroxine concentrations were detected in 35 horses, the majority of those horses had serum thyroxine values within the reference range when retested. Only one horse had a mildly decreased serum tri-iodothyronine concentration. Those horses in which the serum thyroxine concentration was low when retested had a normal thyrotropin releasing hormone stimulation test. Hypothyroidism was not diagnosed in any horses in this study. The low serum thyroxine concentrations measured in the present study were attributed to either normal fluctuations in serum concentrations in healthy horses, the effect of drugs, or to the effects of non-thyroidal illness. Because thyroid hormone concentrations are altered by many factors, hypothyroidism should not be diagnosed on the basis of a single low value and further testing, preferably including active stimulation of the thyroid gland, should be carried out.

Evaluation of an enzymatic immunoassay for free thyroxine determination in canine serum

Free thyroxine (FT4) and cholesterol were measured in 400 dogs with either suspected hypothyroidism or dermatological signs such that hypothyroidism needed to be ruled out. Hypothyroidism was diagnosed in 68 dogs from the history, physical examination and stated lower reference limit (< 7 pmol/L) for FT4 in euthryoid dogs. Dogs with FT4 concentrations in the range 6-9 pmol/L were finally categorized as hypo- or euthyroid either on the basis of retesting after 2 months or on their clinical response to thyroid replacement therapy over at least 2 months. The enzyme immunoassay evaluated in this paper is considered to be of clinical value and offers many advantages compared with radioimmunoassays.

Histochemical study of cutaneous mucins in hypothyroid dogs

A dermal mucinosis, visualized as dermal alcianophilic material, is occasionally present in canine hypothyroidism (myxedema). Various histochemical reactions (alcian blue/periodic acid-Schiff [PAS], alcian blue at pH 2.6, alcian blue at pH 1.0, critical electrolytical concentrations with and without dimethylsulfoxide, differential hydrolysis by hyaluronidases) were performed on skin biopsies from six dogs (four females and two males ranging from 8 to 13 years) affected by hypothyroidism, all of them presenting dermal mucinosis in hematoxylin and eosin-stained sections. In these dogs, the only polysaccharidic compound involved in the dermal mucinosis was hyaluronic acid. In this study, hyaluronic acid dermal deposits of hypothyroid dogs were significantly different from those of controls in subepidermal connective tissue and loose reticular connective tissue but not in periadnexal zones. We recommend the combined alcian blue/PAS reaction as a routine technique to assess dermal mucinosis in hypothyroid dogs.

Neurologic manifestations of thyroid disease

Animals with polyneuropathy associated with primary hypothyroidism have clinical neurologic signs that range from peripheral vestibular signs, lower motor neuronal deficits, laryngeal paralysis, to megaesophagus; however, a few affected animals also show evidence of a more generalized polyneuropathy with cranial (facial more than vestibular nerve) and spinal nerves being affected most commonly. Confirmation of the diagnosis will depend on neurologic signs, clinicopathologic results including an abnormally low T4 response to TSH and electrodiagnostic findings. Successful treatment is based on thyroxine supplementation therapy. In severe cases with laryngeal paralysis, additional surgical treatment is indicated. Some animals with megaesophagus may develop permanent neurologic disabilities.

Update on diagnosis of canine hypothyroidism

Until a clinically useful canine TSH assay is available, a valid test of thyroid hypofunction should (1) take into consideration valid (or at least predictive) measurement of the free T4 concentration in order to factor out the binding effects of drugs and nonthyroidal illness, and (2) should assess thyroid functional reserve. A validated equilibrium dialysis measurement of free T4 and the TSH stimulation test, provide the most useful diagnostic combination of tests now available. A therapeutic trial with L-thyroxine, despite being relatively safe, should be reserved once other diagnoses have been ruled out, and should be reevaluated to assure a clinical response which can be attributable to the medication.

Impaired local deiodination of thyroxine to triiodothyronine in dogs with symmetrical truncal alopecia

Thyroid dysfunction causes certain dermatological alterations in dogs. Insufficient delivery of thyroid hormone to the skin may originate not only from inadequate thyroid function but also from impaired local activation of thyroxine in the target organ. Thyroid parameters and deiodination were investigated in healthy dogs (group C) and in dogs with cutaneous lesions associated with hypothyroidism (group H) or with a low-T3 syndrome (group LT). The ability of the skin to convert T4 to T3 was impaired in both groups H and LT but not in the controls. It is concluded that impaired local deiodination may contribute to skin problems in dogs.

Current diagnostic techniques for evaluating thyroid function in the dog

Hypothyroidism is a common endocrinopathy in dogs. There are a wide variety of diagnostic tests to evaluate thyroid function. The measurement of basal thyroid hormone concentrations is used widely as an indicator of thyroid function; however, there are many factors that may affect basal hormone concentrations. The thyrotropic stimulation test is not affected by many of these factors, and often it is preferred for this reason. Current diagnostic procedures for evaluating thyroid function for the veterinary practitioner are reviewed.

Purification and partial characterization of thyroid hormone binding proteins in canine serum

Thyroxine-binding prealbumin (TBPA) and thyroxine-binding globulin (TBG) were isolated from canine serum and partially characterized. TBPA was isolated by retinol-binding protein (RBP) affinity chromatography and further purified by preparative agarose gel electrophoresis or FPLC ion exchange chromatography. TBG was purified by thyroxine (T4)-Sepharose chromatography followed by gel filtration on Sephacryl S-300 and preparative electrofocusing in a granulated dextran gel. Molecular weights were estimated by SDS-polyacrylamide gradient gel electrophoresis. Canine TBPA had a tetramer molecular weight of 56,000, an extinction coefficient of 12.8 cm2cg.1, an isoelectric point of 5.26-5.70 and a microheterogeneity pattern similar to that of human TBPA. Partial immunochemical identity with human TBPA was also found. Plasma concentrations of TBPA were measured by rocket immunoelectrophoresis in 43 normal and 35 hypothyroid dogs. Reference levels for TBPA ranged between 205 and 474 mg/l. Hypothyroid dogs had a mean TBPA level of 315.0 mg/l (SD: 91.1 mg/l). TBG had a molecular weight of 75,000 and an isoelectric point of 5.0. No immunochemical identity with human TBG was found. Gel filtration of serum on Sephacryl S-200, identification of T4-binding proteins with 125I-T4, and protein- and lipoprotein staining of fractions was performed. Thyroxine-binding was found to TBG in the beta-globulin region, TBPA in the alpha 2-region, albumin, and to the high density lipoprotein (HDL2) in the alpha 1-region and the very low density lipoprotein (VLDL) in the pre-beta region. A corresponding band to the latter protein in serum was masked by TBG and TBPA, and T4-binding in the alpha 1-region was not always seen in serum. Many similarities were found between man and dog regarding TBPA, but not TBG. The differences in structure of TBG may in part be responsible for the low serum T4 levels and rapid T4 metabolism seen in dogs.

Effect of thyrotropin storage on thyroid-stimulating hormone response testing in normal dogs

The stability of reconstituted, refrigerated thyrotropin was evaluated. Thyrotropin (TSH) was reconstituted at the start of the study and stored at 4 degrees C. A TSH stimulation test was performed in eight healthy, euthyroid dogs at weekly intervals for 1 month. In seven of eight dogs, there was no significant difference (P less than 0.05) between the post-TSH T3 concentrations and the post-TSH T4 concentrations for the duration of the study. For one dog, the post-TSH T4 concentration was below the normal post-TSH T4 range following the administration of reconstituted TSH that had been stored 4 weeks. The T3 response to the TSH, however, was normal. This dog responded normally to freshly reconstituted TSH. The results of this study suggest that reconstituted bovine TSH can be stored at 4 degrees C for at least 3 weeks without loss of biologic activity in the dog.