Influence of acetylsalicylic acid and ketoprofen on canine thyroid function tests

Assessment of the likelihood of hypothyroidism in dogs diagnosed with and treated for hypothyroidism at primary care practices: 102 cases (2016-2021)

BACKGROUND

There is a possibility that an incorrect diagnosis of hypothyroidism could be made in euthyroid dogs, and the prevalence of hypothyroidism in the dog population remains unknown.

OBJECTIVES

To retrospectively assess the percentage of dogs diagnosed with, and treated for, hypothyroidism at first opinion practice which are likely to be hypothyroid and require levothyroxine supplementation.

ANIMALS

One hundred two client-owned dogs were included in this study.

MATERIALS AND METHODS

The computerized databases of 7 first opinion practices were searched to identify dogs treated with levothyroxine supplementation. Three European College of Veterinary Internal Medicine-Companian Animals (ECVIM-CA) diplomates independently assigned 1 of 4 clinical assessments to each case as follows: confirmed or likely hypothyroid, hypothyroidism suspected but not confirmed, hypothyroidism considered unlikely, and no reason to suspect hypothyroidism. They commented as to whether or not they thought levothyroxine supplementation was appropriate.

RESULTS

The clinical assessments of "confirmed or likely hypothyroid"; "Hypothyroidism suspected but not confirmed"; "Hypothyroidism considered unlikely"; and "No reason to suspect hypothyroidism" was assigned respectively by Clinician 1 to 38.2%, 5.9%, 3.9%, and 52% of cases, by Clinician 2 to 48%, 22.6%, 22.6%, 6.9% of cases, and by Clinician 3 to 55.9%, 11.8%, 13.7% and 18.6%. Clinician 1, Clinician 2, and Clinician 3 considered levothyroxine supplementation not indicated in 58.8%, 52.9%, and 45.1% of cases, respectively.

CONCLUSION

These results support the concern that hypothyroidism might be overly and incorrectly diagnosed in first opinion practice, and that thyroid function testing should be performed only in those dogs with a high pretest probability of the disease.

Thyroid function tests during nonthyroidal illness syndrome and recovery in acutely ill dogs

BACKGROUND

Nonthyroidal illness syndrome (NTIS) can result in thyroid function test alterations that mimic hypothyroidism. The duration of NTIS-induced changes in dogs is not well-described.

OBJECTIVES

Document alterations in thyroid function tests during NTIS and recovery, and the time necessary for their resolution.

ANIMALS

From 103 dogs sampled, 25 euthyroid dogs with acute, resolvable illness having a low serum total thyroxine (TT4) concentration on admission were analyzed.

METHODS

Prospective observational study. Serum TT4 concentration was measured in 103 dogs within 4 hours of admission. If below the reference interval (RI), subsequent serum samples were obtained every 24 hours from admission until discharge (acute phase) and at 2 weeks and 4 weeks after discharge (recovery phase). Serum samples were submitted for batch measurement of serum TT4, free thyroxine (fT4), total 3,5,3'-triiodothyronine (TT3), and thyroid-stimulating hormone (TSH) concentrations.

RESULTS

In the cohort of dogs analyzed, serum TT4, TT3, and fT4 concentrations were below the RI in 100%, 80%, and 16% at admission; 20%, 80%, and 0% at discharge; 4%, 8%, and 0% at 2 weeks; and 0%, 0%, and 0% at 4 weeks, respectively. Serum TSH concentration was within the RI in 100% at admission and discharge, and above the RI in 4% and 12% at 2 weeks and 4 weeks, respectively.

CONCLUSIONS AND CLINICAL IMPORTANCE

Naturally occurring NTIS in dogs induces alterations in thyroid function tests during acute illness and recovery. Measurement of serum TT4 concentration 2 to 4 weeks after discharge or serum fT4 concentration by ED during illness is recommended for accurate assessment of thyroid function in acutely ill dogs.

Relationship between thyroid function and sex hormones in female German shepherd dogs

BACKGROUND

Several variables influence the serum concentration of thyroid hormones in dogs, including breed, age, drugs, and concurrent diseases. However, data regarding the interaction between thyroid function and the estrous stage of female dogs are limited.

HYPOTHESIS

Estrous stage may influence thyroid function in German Shepherd dogs.

METHODS

Longitudinal, observational, non-randomized cohort study. The dogs were monitored during the complete estrous cycle, and different stages were determined by vaginal cytology. Two blood samples were collected at the beginning and end of each stage to analyze the following: total thyroxine (TT4), free thyroxine (fT4), total triiodothyronine (TT3), free triiodothyronine (fT3), canine thyrotropin (cTSH), progesterone, 17-β-estradiol, triglycerides, and cholesterol concentrations. Hematological and biochemical evaluations were performed at the beginning and end of the study period.

ANIMALS

Seventeen German Shepherds were included, of which 7 were bred during the study period. One dog was excluded for estrus interruption and another for suspected hypothyroidism.

RESULTS

Serum concentrations of T4, fT4, and fT3 were negatively correlated with age. Total thyroxine demonstrated significant changes in serum concentrations between estrous stages, with higher concentrations in estrus and diestrus. Total thyroxine concentrations were positively correlated with progesterone concentrations and negatively correlated with 17-β-estradiol concentrations. Free thyroxine did not show significant variations but was positively correlated with progesterone concentrations. Canine TSH concentrations were positively correlated with 17-β-estradiol concentrations. No significant differences in thyroid hormones and cTSH concentrations were observed between diestrus during pregnancy and pseudopregnancy.

CONCLUSIONS

Different stages of estrus can influence the measurement of TT4 in female dogs.

Influence of medications on thyroid function in dogs: An update

Erroneous thyroid function test results can occur because of drugs that alter thyroid hormone physiology in one or more aspects, including synthesis, secretion, distribution, and metabolism. Research since publication of the last review in the Journal of Veterinary Internal Medicine (JVIM) 20 years ago has evaluated the effects of amiodarone, zonisamide, inhalant anesthetics, clomipramine, trilostane, and toceranib on thyroid function tests in the dog. In addition, recent work on the effects of glucocorticoids, sulfonamides, phenobarbital, and nonsteroidal anti-inflammatory drugs will be reviewed. Awareness of these effects is necessary to avoid misdiagnosis of hypothyroidism and unnecessary treatment.

Recombinant human thyrotropin stimulation test in 114 dogs with suspected hypothyroidism: a cross-sectional study

OBJECTIVE

To evaluate the performance and define cut-offs for the interpretation of a thyroid-stimulating hormone (TSH) stimulation test with a recombinant human TSH dose of 75 μg/dog administered intravenously in dogs with suspected hypothyroidism.

MATERIALS AND METHODS

Cross-sectional study. Medical records of dogs presented for suspected hypothyroidism were retrospectively reviewed. Animals were included if a TSH stimulation test with a recombinant human TSH dose of 75 μg/dog was performed and follow-up was available. Dogs with a post-TSH serum total thyroxine (T4) level of ≥2.2 μg/dL were considered euthyroid. Dogs with a post-TSH T4 level of <2.2 μg/dL were classified as hypothyroid or euthyroid based on follow-up, including response to levothyroxine supplementation. A receiver operating characteristic curve analysis was used to define the performance of the test.

RESULTS

One hundred and fourteen dogs were included. Forty were classified as hypothyroid and 74 as euthyroid. Post-TSH T4 cut-offs of 1.3 and 1.7 μg/dL showed sensitivities of 92.5 and 100% and specificities of 97.3 and 93.2%, respectively. Post-TSH T4 levels of >1.7 μg/dL had a negative predictive value of 100%. Post-TSH T4 levels of <1.3 μg/dL showed a positive predictive value of 94.9%. Area under the ROC curve for post-TSH T4 was 0.99.

CLINICAL SIGNIFICANCE

A TSH stimulation test performed with a recombinant human TSH dose of 75 μg/dog is highly reliable to discriminate between hypothyroid and euthyroid dogs, even in cases of concurrent non-thyroidal illness or administration of medications. A post-stimulation T4 concentration of >1.7 μg/dL is suggestive of normal thyroid function.

Levothyroxine supplementation in hypothyroid bitches during pregnancy

Hypothyroidism is the most common endocrine disease in dogs and has been shown to have a hereditary nature in many breeds. Previous studies have documented decreased fertility in bitches with experimentally-induced hypothyroidism, decreased viability at birth, increased periparturient mortality, and reduced birth weight in pups born to hypothyroid dogs. Hypothyroid women have an increased demand for exogenous thyroxine throughout gestation in order to maintain normal plasma concentrations of thyroid hormones and produce neuropsychologically normal children. This study was performed to determine if pregnancy causes a similar need for increased levothyroxine dosages in dogs to maintain a euthyroid state. Serum was harvested from blood collected from six bitches with experimentally-induced hypothyroidism that were receiving standard thyroid hormone replacement therapy and from four euthyroid control bitches. Thyroid function tests performed on these samples included total thyroxine (T₄), free T₄ (fT₄), thyroid stimulating hormone (TSH), and 3,5,3'-triiodothyronine (T₃). Thyroid function tests were measured from ovulation through the end of pregnancy. All bitches whelped normal litters. Euthyroid bitches had no significant alterations in their hormone concentrations throughout pregnancy. None of the supplemented hypothyroid bitches had clinical signs of hypothyroidism throughout the study. Serum concentrations of T₄ and fT₄ were elevated compared to ovulation at multiple sample points during gestation in supplemented dogs. The results from this study indicate that standard levothyroxine supplementation is adequate to maintain a euthyroid state during pregnancy in experimentally-induced hypothyroid dogs. In addition, there is no evidence that canine thyroid profiles in euthyroid dogs are altered during gestation.

Use of basal and TRH-stimulated plasma growth hormone concentrations to differentiate between primary hypothyroidism and nonthyroidal illness in dogs

BACKGROUND

A low plasma total thyroxine (TT₄ ) concentration in combination with a plasma TSH concentration within reference range does not distinguish between hypothyroidism and nonthyroidal illness (NTI) in dogs. Hypothyroidism is associated with TSH-releasing hormone (TRH)-induced increased release of growth hormone (GH).

HYPOTHESIS

Basal and TRH-induced plasma GH concentrations can be used to distinguish hypothyroid dogs from NTI dogs.

ANIMALS

Twenty-one dogs with signs consistent with hypothyroidism, a low plasma TT₄ concentration, and a plasma TSH concentration within reference interval.

METHODS

Case control study. Thyroid scintigraphy was performed to classify dogs as having hypothyroidism or NTI. All dogs underwent a TRH stimulation test with measurement of plasma concentrations of GH and TSH before and 30 and 45 minutes after IV administration of TRH.

RESULTS

Eleven of the dogs were classified as hypothyroid and 10 as having NTI. Basal plasma GH concentration in the hypothyroid dogs (3.2 μg/l; range, 2.0 to 12.5 μg/l) was significantly higher (p<0.001) than that in the NTI dogs (.73 μg/l; range, .45 to 2.3 μg/l), with minimal overlap, and increased (p=.009) after TRH administration in hypothyroid dogs, whereas it did not change in NTI dogs. At T=45, plasma GH concentrations in hypothyroid dogs and NTI dogs did not overlap. The plasma TSH concentration did not change significantly after TRH administration in hypothyroid dogs, whereas it increased (p<.001) in NTI dogs. At T=45, there was no overlap in percentage TSH increase from baseline between hypothyroid dogs.

CONCLUSIONS AND CLINICAL IMPORTANCE

Measurement of basal plasma GH concentration and concentrations of GH and TSH after TRH stimulation can distinguish between hypothyroidism and NTI in dogs.

Canine hypothyroidism: a review of aetiology and diagnosis

Hypothyroidism is recognised as an important endocrine disorder of dogs, and a frequent differential for numerous presenting complaints. Its diagnosis has never been straight forward as results suggestive of hypothyroidism can occur for a variety of reasons in dogs with normal thyroid function (euthyroid). As a consequence, the accurate investigation of hypothyroidism has been hindered by the potential inclusion of a number of cases not truly hypothyroid. In recent years, the development of newer diagnostic tests, e.g. free thyroxine, canine thyroid stimulating hormone, thyroglobulin autoantibodies, has significantly improved our ability to reliably differentiate hypothyroidism from other clinically similar disorders. This has led to a marked increase in our knowledge of the phenotypic, genotypic and aetiological aspects of this disorder in dogs.

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.

Serum thyroid hormone concentrations and thyroglobulin autoantibodies in trained and non-trained healthy whippets

Numerous factors including non-thyroidal systemic diseases and drug administration can significantly alter canine thyroid function test results. Furthermore, the importance of breed specific variations has probably been underestimated. In this study, total thyroxine (TT4), free thyroxine (FT4), canine endogenous thyroid stimulating hormone (cTSH) serum concentrations and thyroglobulin autoantibodies (TgAA) were determined in a population of healthy whippets and compared to a control group of different breeds. Mean TT4 values were significantly lower in the whippets but no significant differences were seen between whippets and control dogs for FT4 and for cTSH. The prevalence of serum TgAA in the whippets was 2%, and this was not significantly different from the controls. The results suggest a breed variation for TT4, but not for FT4, cTSH and TgAA serum concentrations in whippets. Serum thyroid hormone concentrations were also compared between trained and non-trained whippets and it was concluded that regular training did not seem to have any significant influence.

Effects of deracoxib and aspirin on serum concentrations of thyroxine, 3,5,3′-triiodothyronine, free thyroxine, and thyroid-stimulating hormone in healthy dogs

OBJECTIVE

To evaluate the effects of deracoxib and aspirin on serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), free thyroxine (fT4), and thyroid-stimulating hormone (TSH) in healthy dogs.

ANIMALS

24 dogs.

PROCEDURE

Dogs were allocated to 1 of 3 groups of 8 dogs each. Dogs received the vehicle used for deracoxib tablets (PO, q 8 h; placebo), aspirin (23 to 25 mg/kg, PO, q 8 h), or deracoxib (1.25 to 1.8 mg/kg, PO, q 24 h) and placebo (PO, q 8 h) for 28 days. Measurement of serum concentrations of T4, T3, fT4, and TSH were performed 7 days before treatment (day -7), on days 14 and 28 of treatment, and 14 days after treatment was discontinued. Plasma total protein, albumin, and globulin concentrations were measured on days -7 and 28.

RESULTS

Mean serum T4, fT4, and T3 concentrations decreased significantly from baseline on days 14 and 28 of treatment in dogs receiving aspirin, compared with those receiving placebo. Mean plasma total protein, albumin, and globulin concentrations on day 28 decreased significantly in dogs receiving aspirin, compared with those receiving placebo. Fourteen days after administration of aspirin was stopped, differences in hormone concentrations were no longer significant. Differences in serum TSH or the free fraction of T4 were not detected at any time. No significant difference in any of the analytes was detected at any time in dogs treated with deracoxib.

CONCLUSIONS AND CLINICAL RELEVANCE

Aspirin had substantial suppressive effects on thyroid hormone concentrations in dogs. Treatment with high dosages of aspirin, but not deracoxib, should be discontinued prior to evaluation of thyroid function.

Thyroid Disorders in the Geriatric Patient

Thyroid disorders are common in older pets. They often present a diagnostic challenge, and reaching a definitive diagnosis can be difficult or impossible in some cases. It is important for the veterinary practitioner to be familiar with the historical, physical examination, and clinicopathologic data findings in each of these diseases and to become comfortable with the treatment, monitoring, and prognosis associated with thyroid diseases in geriatric pets.