Evaluation of recombinant human thyroid-stimulating hormone to test thyroid function in dogs suspected of having hypothyroidism

OBJECTIVE: To evaluate the use of recombinant human (rh) thyroid-stimulating hormone (TSH) in dogs with suspected hypothyroidism. ANIMALS: 64 dogs with clinical signs of hypothyroidism. PROCEDURES: Dogs received rhTSH (75 microg/dog, IV) at a dose independent of their body weight. Blood samples were taken before and 6 hours after rhTSH administration for determination of total serum thyroxine (T(4)) concentration. Dogs were placed into 1 of 3 groups as follows: those with normal (ie, poststimulation values indicative of euthyroidism), unchanged (ie, poststimulation values indicative of hypothyroidism; no thyroid gland stimulation), or intermediate (ie, poststimulation values between unchanged and normal values) post-TSH T(4) concentrations. Serum canine TSH (cTSH) concentration was determined in prestimulation serum (ie, before TSH administration). RESULTS: 14, 35, and 15 dogs had unchanged, normal, and intermediate post-TSH T(4) concentrations, respectively. Basal T(4) and post-TSH T(4) concentrations were significantly different among groups. On the basis of basal serum T(4) and cTSH concentrations alone, 1 euthyroid (normal post-TSH T(4), low basal T(4), and high cTSH concentrations) and 1 hypothyroid dog (unchanged post-TSH T(4) concentration and low to with-in reference range T(4) and cTSH concentrations) would have been misinterpreted as hypothyroid and euthyroid, respectively. Nine of the 15 dogs with intermediate post-TSHT(4) concentrations had received medication known to affect thyroid function prior to the test, and 2 of them had severe nonthyroidal disease. CONCLUSIONS AND CLINICAL RELEVANCE: The TSH-stimulation test with rhTSH is a valuable diagnostic tool to assess thyroid function in selected dogs in which a diagnosis of hypothyroidism cannot be based on basal T(4) and cTSH concentrations alone.     

Evaluation of serum free thyroxine and thyrotropin concentrations in the diagnosis of canine hypothyroidism

Canine thyroid-stimulating hormone (cTSH), total thyroxine (T4) and free T4 by equilibrium dialysis (fT4d) were measured in serum samples from 107 dogs with clinical signs suggestive of hypothyroidism in which the diagnosis was either confirmed (n = 30) or excluded (n = 77) by exogenous TSH response testing. Median serum total T4 and fT4d concentrations were significantly lower and cTSH significantly higher (P < 0.001) in hypothyroid compared with euthyroid dogs. Differential positive rate analysis determined optimal cut-off values of less than 14.9 nmol/litre (total T4), less than 5.42 pmol/litre (fT4d), greater than 0.68 ng/ml (cTSH), less than 17.3 (T4 to cTSH ratio), and less than 7.5 (fT4d to cTSH ratio) for hypothyroidism. These had a sensitivity and specificity of 100 and 75.3 per cent, 80 and 93.5 per cent, 86.7 and 81.8 per cent, 86.7 and 92.2 per cent, and 80 and 97.4 per cent, respectively, for diagnosing hypothyroidism. Corresponding areas under the receiver operating characteristic curves were 0.92, 0.93, 0.87, 0.93 and 0.93. Unexpectedly low cTSH values in hypothyroid dogs may have resulted from concurrent non-thyroidal illness. Unexpectedly high serum cTSH values in the euthyroid dogs might have resulted from recovery from illness or concurrent potentiated sulphonamide therapy. Measurement of endogenous cTSH concentration is a valuable diagnostic tool for canine hypothyroidism if used in association with assessment of T4. Estimation of fT4d added only limited additional information over total T4 measurement.     

Endogenous TSH in the diagnosis of hypothyroidism in dogs

To determine whether measurement of canine thyrotropin (cTSH) would aid in the diagnosis of hypothyroidism, serum samples of 65 dogs with clinical signs suggestive of hypothyroidism were evaluated. Diagnosis was confirmed in 26 dogs and excluded in 39 dogs based on TSH-stimulation testing. Total thyroxine (T4) was significantly lower and cTSH significantly higher in hypothyroid dogs compared to euthyroid dogs. Canine TSH was above (> 0.6 ng/ml) in 15 (57.7%) and below the upper limit of the reference range in 11 (42.3%) of the hypothyroid dogs. All of the euthyroid dogs had a cTSH < 0.6 ng/ml. In all dogs with a cTSH above the upper limit of the reference range hypothyroidism could be confirmed. Therefore, our results show that measurement of cTSH has an excellent specificity (100%) and is a valuable tool in confirming canine hypothyroidism. However, due to the low sensitivity of cTSH assays (60%), it can not be recommended to exclude the disease.     

Use of recombinant human thyroid-stimulating hormone for thyrotropin stimulation test in healthy, hypothyroid and euthyroid sick dogs

Recombinant human thyroid-stimulating hormone (rhTSH) was evaluated for the diagnosis of canine hypothyroidism, using TSH response tests. Phase I stimulation tests were performed in 6 healthy dogs weighing over 20 kg, using 50 and then 100 microg of freshly reconstituted rhTSH administered intravenously. In phase II, the same dogs were stimulated by using 100 microg of rhTSH frozen for 3 months at -20 degrees C. Phase III stimulation tests were performed by using 50 or 100 microg of freshly reconstituted or frozen rhTSH in healthy (n = 14), euthyroid sick (n = 11) and hypothyroid dogs (n = 9). A dose of 100 microg of rhTSH was judged more appropriate for dogs weighing more than 20 kg. Biological activity of rhTSH after freezing at -20 degrees C for up to 12 weeks was maintained. When stimulated, significant (P < 0.05) increases in total thyroxine concentration were observed only in healthy and euthyroid sick dogs. Results of this study show that the rhTSH stimulation test is able to differentiate euthyroidism from hypothyroidism in dogs.     

Thyroid sonography as an effective tool to discriminate between euthyroid sick and hypothyroid dogs

The diagnosis of canine hypothyroidism and its differentiation from euthyroid sick syndrome still is a major diagnostic challenge. In this study, ultrasonography was shown to be an effective tool for the investigation of thyroid gland diseases. Healthy control dogs (n = 87), dogs with euthyroid sick syndrome (n = 26), thyroglobulin autoantibody-positive (TgAA-positive, n = 30) hypothyroid dogs, and TgAA-negative (n = 23) hypothyroid dogs were examined by thyroid ultrasonography. Maximal cross sectional area (MCSA), thyroid volume, and echogenicity were measured. Statistical analysis identified highly significant (P < .001) differences between euthyroid and hypothyroid dogs both in thyroid volume and in MCSA, whereas no significant differences in thyroid size were detected between healthy euthyroid dogs and dogs with euthyroid sick syndrome. In euthyroid and euthyroid sick dogs, parenchymal echotexture was homogeneous and hyperechoic, whereas relative thyroid echogenicity of both TgAA-positive and TgAA-negative hypothyroid dogs was significantly lower (P < .001). When using arbitrarily chosen cutoff values for relative thyroid volume, MCSA, and echogenicity, thyroid volume especially was found to have highly specific predictive value for canine hypothyroidism. In summary, the data reveal that thyroid sonography is an effective ancillary diagnostic tool to differentiate between canine hypothyroidism and euthyroid sick syndrome.     

Tertiary hypothyroidism in a dog

: A nine-year-old male entire Labrador was diagnosed with pituitary dependent hyperadrenocorticism. Following seven months of successful mitotane therapy, the dog presented with marked weight gain, seborrhoea and alopecia. Routine clinicopathological analyses revealed marked hypercholesterolaemia. Serum total and free thyroxine (T4) concentrations were below their respective reference ranges. Serum thyroid stimulating hormone (cTSH) concentration was within reference range. TSH and thyrotropin releasing hormone (TRH) response tests revealed adequate stimulation of total T4 in both, and cTSH in the latter test. Magnetic resonance imaging revealed a mass arising from the pituitary fossa, with suprasellar extension. A diagnosis of tertiary hypothyroidism was made. Following four weeks of levothyroxine therapy, circulating cholesterol concentration had declined, weight loss had ensued and dermatological abnormalities had improved. Euthanasia was performed four months later due to the development of neurological signs. A highly infiltrative pituitary adenoma, with effacement of the overlying hypothalamus was identified on post mortem examination. Tertiary hypothyroidism has not been previously reported in dogs.     

Is it possible to diagnose canine hypothyroidism?

A definitive diagnosis of hypothyroidism can be difficult because of the many clinical abnormalities associated with thyroid hormone deficiency, and the lack of readily available diagnostic tests with high sensitivity and specificity. Thyroid function tests should be performed only in dogs with clinical findings consistent with hypothyroidism. Measurement of serum total thyroxine (T4) concentration is a useful initial screening test since most hypothyroid dogs have values below the reference range. Serum free T4 concentration measured by equilibrium dialysis is a more sensitive and specific test of thyroid function than total T4 and is particularly useful in dogs with non-thyroidal illness or atypical clinical signs. Measurement of serum endogenous thyroid-stimulating hormone concentration is also helpful, but many hypothyroid dogs have normal results. The gold standard for diagnosis of hypothyroidism remains the thyroid-stimulating hormone response test. It should be used to confirm hypothyroidism when other tests do not agree with the clinical impression or if atypical signs or non-thyroidal illness exist or there has been administration of drugs known to alter thyroid function tests. Ultimately, a positive response to treatment is expected in hypothyroid dogs treated appropriately with levothyroxine.     

Serum-free thyroxine concentrations, measured by chemiluminescence assay before and after thyrotropin administration in healthy dogs, hypothyroid dogs, and euthyroid dogs with dermathopathies.

The purpose of this study was to determine the usefulness of free thyroxine (FT4) measured by chemiluminescence in evaluating thyroid function in dogs. Total thyroxine (TT4) concentration measured by radioimmunoassay (RIA) and FT4 measured by chemiluminescence were evaluated in 30 healthy dogs, 60 euthyroid dogs with concurrent dermatopathies, and 30 hypothyroid dogs before and after intravenous stimulation with 1 or 2 IU of thyrotropin (TSH). Median basal TT4 and median TT4 concentrations at 4 h post-TSH administration were not significantly different (P < 0.0001) between healthy dogs and euthyroid dogs with dermatopathies, but were significantly higher than those in hypothyroid dogs. In healthy dogs, the median TT4 concentrations at 4 and 6 h post-TSH administration were not significantly different. Median basal FT4 and median FT4 concentrations at 4 h post-TSH administration in healthy dogs were significantly lower (P < 0.0001) than those in euthyroid dogs with dermatopathies, but significantly higher than the same parameters in hypothyroid dogs. There was a significant difference between the median FT4 concentrations at 4 h post-TSH administration and median basal FT4 concentrations for healthy dogs and euthyroid dogs with dermatopathies, but not for hypothyroid dogs. Lastly, in healthy dogs, median FT4 concentrations at 4 and 6 h post-TSH administration were not significantly different. Free thyroxine measured by chemiluminescence was highly correlated (P < 0.0001; Spearman r = 0.91) with FT4 measured by the reference method for free hormone analysis, namely, equilibrium dialysis, when sera from 56 dogs were used.     

Thyroid function testing in Greyhounds.

OBJECTIVE: To evaluate thyroid function in healthy Greyhounds, compared with healthy non-Greyhound pet dogs, and to establish appropriate reference range values for Greyhounds. ANIMALS: 98 clinically normal Greyhounds and 19 clinically normal non-Greyhounds. PROCEDURES: Greyhounds were in 2 groups as follows: those receiving testosterone for estrus suppression (T-group Greyhounds) and those not receiving estrus suppressive medication (NT-group Greyhounds). Serum thyroxine (T4) and free thyroxine (fT4) concentrations were determined before and after administration of thyroid-stimulating hormone (TSH) and thyroid-releasing hormone (TRH). Basal serum canine thyroid stimulating hormone (cTSH) concentrations were determined on available stored sera. RESULTS: Basal serum T4 and fT4 concentrations were significantly lower in Greyhounds than in non-Greyhounds. Serum T4 concentrations after TSH and TRH administration were significantly lower in Greyhounds than in non-Greyhounds. Serum fT4 concentrations after TSH and TRH administration were significantly lower in NT-group than T-group Greyhounds and non-Greyhounds. Mean cTSH concentrations were not different between Greyhounds and non-Greyhounds. CONCLUSIONS AND CLINICAL RELEVANCE: Previously established canine reference range values for basal serum T4 and fT4 may not be appropriate for use in Greyhounds. Greyhound-specific reference range values for basal serum T4 and fT4 concentrations should be applied when evaluating thyroid function in Greyhounds. Basal cTSH concentrations in Greyhounds are similar to non-Greyhound pet dogs.     

Effects of sulfamethoxazole-trimethoprim on thyroid function in dogs

OBJECTIVE: To evaluate effects of trimethoprim-sulfamethoxazole (T/SMX) on thyroid function in dogs. ANIMALS: 6 healthy euthyroid dogs. PROCEDURE: Dogs were administered T/SMX (14.1 to 16 mg/kg, PO, q 12 h) for 3 weeks. Blood was collected weekly for 6 weeks for determination of total thyroxine (TT4), free thyroxine (fT4), and canine thyroid-stimulating hormone (cTSH) concentrations. Schirmer tear tests were performed weekly. Blood was collected for CBC prior to antimicrobial treatment and at 3 and 6 weeks. RESULTS: 5 dogs had serum TT4 concentrations equal to or less than the lower reference limit, and 4 dogs had serum fT4 less than the lower reference limit after 3 weeks of T/SMX administration; cTSH concentrations were greater than the upper reference limit in 4 dogs. All dogs had TT4 and fT4 concentrations greater than the lower reference limit after T/SMX administration was discontinued for 1 week, and cTSH concentrations were less than reference range after T/SMX administration was discontinued for 2 weeks. Two dogs developed decreased tear production, which returned to normal after discontinuing administration. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that administration of T/SMX at a dosage of 14.1 to 16 mg/kg, PO, every 12 hours for 3 weeks caused decreased TT4 and fT4 concentrations and increased cTSH concentration, conditions that would be compatible with a diagnosis of hypothyroidism. Therefore, dogs should not have thyroid function evaluated while receiving this dosage of T/SMX for >2 weeks. These results are in contrast to those of a previous study of trimethoprim-sulfadiazine.     

Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 microg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration < 2 nmol/l and plasma thyrotropin (TSH) concentration > 1 microg/l. In addition, (99m)TcO(4)(-) uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3+/-0.5 microg/l) and increased significantly (P=0.001) 10 and 20 min after injection of TRH (to 11.9+/-3.5 and 9.8+/-2.7 microg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3+/-0.1 microg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.     

Effect of 131I-induced hypothyroidism on indices of reproductive function in adult male dogs

Hypothyroidism has been cited as a cause of infertility, abnormal semen quality, and poor libido in people and animals. The purpose of this study was to evaluate the effect of hypothyroidism on variables indicative of reproductive function in adult male dogs. Nine normal dogs were randomly assigned to 2 groups. Hypothyroidism was induced with 131I in 6 dogs. Three dogs remained untreated, normal, and euthyroid. Thyroid hormone concentrations, body weight, clinical signs, and reproductive function were determined for each dog every 3 months for 2 years. Reproductive function was assessed by determining daily sperm output, total scrotal width, spermatozoal motility and morphology, libido, and serum testosterone and luteinizing hormone concentration responses to exogenous gonadotropin-releasing hormone. The 131I-treated dogs developed clinical and laboratory signs of hypothyroidism. In the hypothyroid dogs, serum concentrations of thyroid hormones were consistently below the reference range and were significantly lower than that in the euthyroid dogs. There was no difference in reproductive function between the hypothyroid and euthyroid dogs. The results of this study show that 131I-induced hypothyroidism does not affect indices of reproductive function in adult male dogs.     

Secretion pattern of thyroid-stimulating hormone in dogs during euthyroidism and hypothyroidism

In as many as one third of dogs with primary hypothyroidism a plasma thyrotropin (TSH) concentration within the reference range for euthyroid dogs is found. To determine whether this is due to fluctuations in the release of TSH, the plasma profiles of TSH were analyzed in 7 beagle bitches by collecting blood samples every 10 min for 6 hr, both before and after induction of primary hypothyroidism. After induction of primary hypothyroidism, a 37-fold increase in mean basal plasma TSH concentration and a 34-fold increase in mean area under the curve for TSH were found. Analysis by the Pulsar program demonstrated pulsatile secretion of TSH in the hypothyroid state, characterized by relatively low amplitude pulses (mean [+/-SEM]) amplitude 41 +/- 3% of basal plasma TSH level) and a mean pulse frequency of 2.0 +/- 0.5 pulses/6 hr. In the euthyroid state, significant TSH pulses were identified in only 2 dogs. The mean basal plasma TSH level correlated positively (r = 0.84) with the mean amplitude of the TSH pulses, and correlated negatively (r = -0.88) with the TSH pulse frequency. The results of this study demonstrate pulsatile secretion of TSH in dogs during hypothyroidism and only small fluctuations in plasma TSH concentrations during euthyroidism. The findings also suggest that the low TSH values occasionally found in dogs with spontaneous primary hypothyroidism may in some cases in part be the result of ultradian fluctuations.     

Clinical,hematological, biochemical and endocrinological aspects of 32 dogs with hypothyroidism

During the years of 1996-2001, hypothyroidism was diagnosed at the clinic for small animal internal medicine, University of Zurich, in 32 dogs. Most of the dogs were large breeds. The most frequent clinical characteristics observed were exercise intolerance, obesity, dermatological, neurological and gastrointestinal signs. Predominant laboratory abnormalities were a low red blood cell count, increased concentration of cholesterol, triglycerides and fructosamin. 29 dogs had a T4 below the reference range (< 1.5 micrograms/dl), one dog had a T4 at the lower limit thereof (1.6 micrograms/dl). One dog had a T4 within the reference range (3.4 micrograms/dl), another had a very high T4 of 206.8 micrograms/dl; the results of the latter 2 dogs were interpreted as incorrectly increased T4 values due to in vitro interference with T4-autoantibodies. Diagnosis was confirmed in all of the dogs based on TSH-stimulation testing. Endogenous TSH (cTSH) measured parallelly, was elevated in only 60% of the dogs. In about 67% of the dogs, hypothyroidism was associated with thyroglobulin-autoantibodies. Canine hypothyroidism is a rather rare endocrine disorder in Switzerland. The TSH-stimulation test remains the gold standard in confirming the disease; a definitive diagnosis can be challenging for practitioners because bovine TSH, used for the TSH-stimulation test is not licensed for use in dogs. Since assessment of cTSH using current assays shows normal values in a high percentage of hypothyroid dogs, the diagnostic value is only limited. In most of the hypothyroid dogs T4 is decreased, with the presence of autoantibodies to T4, it can be normal or increased.     

Exercise-induced hyperkalemia in hypothyroid dogs

We investigated the effect of hypothyroidism in dogs on (1) the Na+-, K+ -ATPase concentration in skeletal muscle, and (2) potassium (K+) homeostasis at rest and during exercise. Prior to and 1 year after induction of hypothyroidism by surgery and subsequent radiothyroidectomy, the Na+-, K+ -ATPase concentrations were quantified in biopsies of sternothyroid muscles of seven Beagle dogs by measuring [3H]ouabain binding capacity. In addition, plasma K+ concentrations were measured at rest and after treadmill exercise in six hypothyroid and seven euthyroid Beagle dogs. During hypothyroidism, the mean Na+ -, K+ -ATPase concentration in muscle biopsies was 41% lower than during euthyroidism. The mean resting plasma K+ value of the hypothyroid dogs was significantly (14%) higher than that of the euthyroid dogs. In the hypothyroid dogs, plasma K+ concentration increased significantly during exercise, whereas there was no rise in the euthyroid dogs. The rise in plasma K+ concentration could not be ascribed to muscle damage, as plasma creatine kinase concentrations remained within reference range. Also renal K+ retention was an unlikely explanation, as plasma aldosterone concentration and plasma renin activity rather increased than decreased during exercise. In conclusion, hypothyroid dogs tend to develop hyperkalemia during exercise, which for a large part can be explained by the severe reduction of the Na+ -, K+ -ATPase capacity in the skeletal muscle pool.     

Thyroid function tests in euthyroid dogs treated with L-thyroxine

The effects of treatment with L-thyroxine (1 mg/m2 of body surface/d, PO, for 8 weeks) on the thyroxine (T4) and triiodothyronine (T3) responses to thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) administration were determined in 10 euthyroid Beagles; 4 other dogs acted as controls. The TSH response test was performed before treatment and at weeks 2, 4, and 8 of treatment in all dogs and at 2 and 4 weeks after cessation of treatment in 6 dogs. The TRH response test was performed before treatment and at week 6 of treatment in all dogs and at 5 weeks after cessation of treatment in 6 dogs. Suppression of the T3 response to TSH was evident at treatment week 2, whereas the T4 response was suppressed at week 4 and remained suppressed for the duration of the study. Four weeks after stopping treatment, T4 and T3 responses to TSH in 2 dogs were within the hypothyroid range. The T4 response to TRH was completely suppressed after 6 weeks of thyroxine treatment, but returned to pretreatment values by 5 weeks after cessation of treatment. Suppression of thyroid and pituitary function is evident after administration of a replacement dose of L-thyroxine to euthyroid dogs.     

Serial thyroid hormone concentrations in healthy euthyroid dogs, dogs with hypothyroidism, and euthyroid dogs with atopic dermatitis.

Serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) concentrations were determined every 3 h for 12 h beginning at 8 a.m. in 20 healthy euthyroid dogs, 19 dogs with hypothyroidism, and 18 euthyroid dogs with atopic dermatitis. Status of thyroid function was based on history, physical findings, results of thyrotropin response testing, and requirement for thyroid hormone replacement therapy. Mean serum T4 and T3 concentrations did not vary significantly between blood samplings within each of the three groups of dogs. Between groups of dogs, mean serum T4 concentration was significantly (P less than 0.05) higher at each blood sampling time in healthy euthyroid dogs and euthyroid dogs with atopic dermatitis when compared to dogs with hypothyroidism. There was no significant difference in mean serum T4 concentration at any blood sampling time between healthy euthyroid dogs and euthyroid dogs with atopic dermatitis or in mean serum T3 concentrations at any blood sampling time between any of the three groups of dogs. Random fluctuation in serum T4 and T3 concentrations was found in dogs in all three groups. Random fluctuations were more common with serum T3 versus T4 concentrations. Consequently, sensitivity (0.88 versus 0.52), specificity (0.73 versus 0.45), predictive value for a positive test (0.75 versus 0.32), predictive value for a negative test (0.87 versus 0.65), and accuracy (0.80 versus 0.47) were better for serum T4 concentration than serum T3 concentration, respectively, when all blood samples were analysed. Measurement of serum T4 concentration was more accurate than serum T3 concentration in assessing the status of thyroid gland function.     

Serum thyroid hormone concentrations and thyrotropin responsiveness in dogs with generalized dermatologic disease.

Fifty-eight dogs with generalized dermatologic disease that had not been given glucocorticoids systemically or topically within 6 weeks of entering the study were evaluated for thyroid function by use of the thyrotropin-response test. Dogs were classified as euthyroid or hypothyroid on the basis of test results and response to thyroid hormone replacement therapy. Baseline serum thyroxine (T4), free T4 (fT4), and triiodothyronine (T3) concentrations were evaluated in the 58 dogs. Serum T4, fT4, and T3 concentrations were evaluated in 200 healthy dogs to establish normal values. Hormone concentrations were considered low if they were less than the mean -2 SD of the values for control dogs. Specificity of T4 and fT4 concentrations was 100% in predicting hypothyroidism; none of the euthyroid dogs with generalized skin disease had baseline serum T4 or fT4 concentration in the low range. Sensitivity was better for fT4 (89%) than for T4 (44%) concentration. Significant difference was not observed in serum T4 and fT4 concentrations between euthyroid dogs with generalized skin disease and healthy control dogs without skin disease. Serum T3 concentration was not accurate in predicting thyroid function; most of the euthyroid and hypothyroid dogs with skin disease had serum T3 concentration within the normal range.     

Treatment and therapeutic monitoring of canine hypothyroidism

Thirty-one dogs with spontaneous hypothyroidism were treated with thyroid hormone replacement therapy (THRT) and monitored for approximately three months. Good clinical and laboratory control was ultimately achieved in all cases with a mean L-thyroxine (T4) dose of 0.026 mg/kg administered once daily. There was a significant increase and decrease in circulating total T4 and canine thyroid stimulating hormone (cTSH) concentrations, respectively, after starting THRT. After commencing treatment, 11 cases subsequently required an increase and three cases required a decrease in dose to achieve optimal clinical control. Median (semi interquartile range [SIR]) circulating six-hour post-pill total T4 (53.6 [27.91 nmol/litre) and cTSH (0.03 [0] microg/litre) concentrations were significantly increased and decreased, respectively, in treated dogs that did not require a dose change; corresponding values in treated dogs in which an increase in dose was required were 29.3 (12.7) nmol/litre and 0.15 (0.62) microg/litre, respectively. However, circulating cTSH measurement was of limited value in assessing therapeutic control because, although increased values were associated with inadequate therapy, reference range cTSH values were common in inadequately treated dogs. Lethargy and mental demeanour were typically the first clinical signs to improve, with significant bodyweight reduction occurring within two weeks of commencing THRT. Routine clinicopathological monitoring was of value in confirming a general metabolic response to THRT, but was of limited value in accurately monitoring cases or tailoring therapy in individual cases.     

Association of Iatrogenic Hypothyroidism with Azotemia and Reduced Survival Time in Cats Treated for Hyperthyroidism

Background: Iatrogenic hypothyroidism can occur after treatment of hyperthyroidism, and is correlated with a reduced glomerular filtration rate in humans and dogs. Hypothesis: Cats with iatrogenic hypothyroidism after treatment for hyperthyroidism will have a greater incidence of azotemia than euthyroid cats. Animals: Eighty client owned cats with hyperthyroidism. Methods: Two retrospective studies. (1) Longitudinal study of 12 hyperthyroid cats treated with radioiodine (documented as euthyroid after treatment), to assess changes in plasma thyroid stimulating hormone (TSH) concentration over a 6‐month follow‐up period, (2) Cross‐sectional study of 75 hyperthyroid cats (documented as euthyroid) 6 months after commencement of treatment for hyperthyroidism to identify the relationship between thyroid status and the development of azotemia. Kaplan‐Meier survival analysis was performed to identify relationships between thyroid and renal status and survival. Results: Plasma TSH concentrations were not suppressed in 7 of 8 cats with hypothyroidism 3 months after radioiodine treatment. The proportion of cats with azotemia was significantly (P= .028) greater in the hypothyroid (16 of 28) than the euthyroid group (14 of 47). Twenty‐eight of 41 cats (68%) with plasma TT4 concentration below the laboratory reference range had an increased plasma TSH concentration. Hypothyroid cats that developed azotemia within the follow‐up period had significantly (P= .018) shorter survival times (median survival time 456 days, range 231–1589 days) than those that remained nonazotemic (median survival time 905 days, range 316–1869 days). Conclusions and Clinical Importance: Iatrogenic hypothyroidism appears to contribute to the development of azotemia after treatment of hyperthyroidism, and reduced survival time in azotemic cats.