Use of the Thyrotropin Releasing Hormone Stimulation Test to Diagnose Mild Hyperthyroidism in Cats

We evaluated serum T₄ and T₃ concentrations before and after administration of thyrotropin releasing hormone (TRH) in 35 cats with mild to moderate hyperthyroidism. 15 cats with nonthyroidal disease, and 31 clinically normal cats. The TRH stimulation test was performed by collecting blood for serum T₄ and T₃ determinations before and 4 hours after IV administration of 0.1 mg/kg TRH. Mean basal serum thyroid hormone concentrations in hyperthy-roid cats were significantly (P < .05) higher than concentrations in normal cats and in those with nonthyroidal disease, but there was considerable overlap among the 3 groups. After administration of TRH, mean serum T₄ concentrations increased significantly in all groups of cats, whereas mean T₃ concentrations increased significantly in normal cats and in those with nonthyroidal disease, but not in cats with hyperthyroidism. The absolute difference between mean basal and TRH-stimulated serum concentrations of T₄ in cats with hyperthyroidism (10.7 nmol/L) was significantly lower than the difference in the cats with nonthyroidal disease (20.0 nmol/L) and in clinically normal cats (28.3 nmol/L), but there was considerable overlap in values among groups. The mean value for relative change in serum T₄ concentration after TRH was significantly lower incats with hyperthyroidism (18.9%) than in those with nonthyroidal disease (110.0%) and in clinically normal cats (130.2%). Serum T₄ concentrations increased by > 50% in all normal cats and cats with nonthyroidal disease, whereas only 4(11.4%) of the 35 hyperthyroid cats had an increase of > 50% after TRH administration. On the basis of canonical discriminate analysis, the mean discriminant function score was significantly higher in the hyperthyroid cats (D = 63.8) than in cats with nonthyroidal disease (D = 5.9) or clinically normal cats (D = 0.7). All cats having a discriminant function score > 30 were hyperthyroid, whereas all cats with a value < 20 were euthyroid. Adverse side effects associated with administration of TRH were common and included transient vomiting, salivation, tachypnea, and defecation. Results of this study indicate that the TRH stimulation test is a useful aid in the diagnosis of hyperthyroidism in cats when basal serum T₄ concentrations are high-normal or only slightly high. As a diagnostic test, the TRH stimulation test compares favorably with the T₃ suppression test but requires less time and is more convenient to perform.     

EFFECTS OF METHIMAZOLE ON THYROID GLAND UPTAKE OF 99MTC-PERTECHNETATE IN 19 HYPERTHYROID CATS

Nineteen cats with abnormally high serum T4 concentrations underwent thyroid scintigraphy using technetium-99m pertechnetate (99mTcO4) before and after 36 +/- 6 days of methimazole administration (approximately 2.5mg PO q 12 h). Thyroid-to-salivary gland ratios (T:S ratios) and percentage thyroidal uptake of injected radioactivity at 20 and 60min after injection of 99mTcO4 were compared before and after methimazole treatment. Serum thyroid stimulating hormone (TSH) concentration was measured before and after methimazole treatment. Quantitatively, there was a positive association between the thyroid uptake of 99mTcO4 and the serum T4 before treatment (r = 0.74-0.83). TSH suppression was present when cats were first evaluated for hyperthyroidism. Methimazole treatment did not relieve TSH suppression in 17 cats. Two cats with unilateral thyroid uptake developed bilateral, asymmetric thyroid uptake of 99mTcO4 after treatment and had the greatest increase in TSH concentration after treatment. Quantitatively, thyroid scintigraphy did not significantly change after methimazole treatment (P>0.1). Evaluation of serum TSH concentration may be helpful in identifying methimazole-induced changes in the scintigraphic features of hyperthyroidism in mildly hyperthyroid cats.     

Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease

OBJECTIVE: To determine the usefulness of measuring serum free thyroxine (T4) concentration as a diagnostic test for hyperthyroidism in cats, and to determine the influence of nonthyroidal disease on free T4 concentration in cats without hyperthyroidism. DESIGN: Prospective case series. ANIMALS: 917 cats with untreated hyperthyroidism, 221 cats with nonthyroidal disease, and 172 clinically normal cats. PROCEDURE: Serum free T4, total T4, and total triiodothyronine (T3) concentrations were measured in cats with untreated hyperthyroidism and cats with nonthyroidal disease. Serum total T4 and T3 concentrations were determined by use of radioimmunoassay, and free T4 concentration was measured by use of direct equilibrium dialysis. Reference ranges for hormone concentrations were established on the basis of results from the 172 clinically normal cats. RESULTS: Sensitivity of serum free T4 concentration as a diagnostic test for hyperthyroidism was significantly higher than the test sensitivity of either total T4 or T3 concentration. Of the 221 cats with nonthyroidal disease, 14 had a high free T4 concentration (ie, false-positive result). Therefore, calculated specificity of measuring serum free T4 concentration as a diagnostic test for hyperthyroidism was significantly lower than test specificity of measuring either the total T4 or T3 concentration. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that determination of free T4 concentration is useful in the diagnosis of hyperthyroidism, especially in cats in which hyperthyroidism is suspected but total T4 and T3 concentrations are within reference ranges. However, because some cats with nonthyroidal disease have high serum free T4 concentrations, hyperthyroidism should not be diagnosed solely on the finding of high free T4 concentration.     

INVESTIGATION OF TWO METHODS FOR ASSESSING THYROID-LOBE ASYMMETRY DURING PERTECHNETATE SCINTIGRAPHY IN SUSPECTED HYPERTHYROID CATS

Our aim was to investigate thyroid:thyroid (T:T) ratio and visual inspection for assessing thyroid-lobe asymmetry in suspected hyperthyroid cats. Although thyroid-salivary asymmetry is a preferred test, inherent thyroid symmetry may assist image interpretation. Association was determined using a scatter plot and Spearman's rank correlation. Agreement was assessed using the kappa (K) statistic. Accuracy was assessed by sensitivity and specificity. Hyperthyroidism was diagnosed in 33/48 (69%) cats based on elevated serum total thyroxine level. Using two Wilcoxan rank-sum tests, a significant difference (P < 0.0001) was detected between cats with and without hyperthyroidism for both methods of assessing thyroid symmetry. For the 18 cats with T:T ratios < or = 1.5, there was poor correlation between the two methods (r(s) = 0.39). Using a cut-point of 1.5 for the T:T ratio, the test accurately predicted hyperthyroidism in 28/33 cats (sensitivity, 85%; 95% confidence interval (CI), 71-99%) and correctly predicted that hyperthyroidism was absent in 14/15 cats (specificity, 93%; CI, 77-100%). For visual inspection, agreement for diagnosing hyperthyroidism was excellent between methods (kappa = 0.82), within the same examiner (weighted kappa = 0.85) and between examiners (weighted kappa = 0.89). Considering cats with only definitely asymmetric thyroid lobes as positive, visual inspection accurately predicted hyperthyroidism in 28/33 cats (sensitivity, 85%; CI, 71-99%) and correctly predicted that hyperthyroidism was absent in 11/15 cats (specificity, 73%; CI, 48-99%). Thyroid-lobe asymmetry occurs more frequently in hyperthyroid than in euthyroid cats but caution should be exercised because some euthyroid cats have asymmetric thyroid glands.     

Serum Thyroid Hormone Concentrations Fluctuate in Cats with Hyperthyroidism

We measured serum thyroxine (T4) and 3,3',5-triiodothyronine (T3) concentrations in hyperthyroid cats (hourly for 10 hours in 14 cats, and daily for 15 days in seven cats) to assess fluctuation in thyroid hormone levels. Over the 10-hour study period the coefficient of variation (CV) for serum T4 and T3 concentrations ranged from 6.4-22.6% (mean = 12.0 +/- 4.8%) and from 9.6-33.1% (mean = 17.5 +/- 6.3%), respectively. During the 15-day study period, CV for serum T4 ranged from 6.6-34.8% (mean = 18.4 +/- 9.3%), while CV for serum T3 ranged from 7.8-31.0% (mean = 20.1 +/- 8.6%). These CV values were significantly higher than the expected intra-assay CVs (T4 assay, 5.1%; T3 assay, 7.7%). In addition, some of the cats with mild hyperthyroidism showed one or more normal serum T4 and T3 values during the course of the respective study periods. There was no specific time during the 10-hour study period at which the cats consistently showed peak serum T4 or T3 concentrations. These results suggest that serum thyroid hormone concentrations are subject to a degree of fluctuation that exceeds the usual assay variation, and that cats with mild hyperthyroidism can, at a given time, exhibit normal serum T4 and T3 values. Therefore, a diagnosis of feline hyperthyroidism should not be excluded on the basis of the finding of a single normal serum T4 or T3 value in a cat with clinical signs and physical examination findings consistent with the disease.     

Effects of dietary selenium and moisture on the physical activity and thyroid axis of cats

Consumption of canned cat food is considered a risk factor for the development of feline hyperthyroidism. Because selenium and water are substantially higher in canned diets compared to dry diets, objectives of this study were to determine whether increased dietary selenium or water alters the function of the hypothalamic–pituitary–thyroid axis and leads to an increase in activity level. Employing a 28‐day latin square design with a 14‐day washout, six lean, neutered male domestic shorthair cats were fed (i) commercially available adult dry feline diet containing 0.8 ppm selenium (control), (ii) control diet with added sodium selenite to achieve a dietary selenium concentration of 1.125 ppm (selenium treatment) and (iii) the control diet with additional water to achieve a moisture content of 75% wt/wt (water treatment). Water consumption was determined using deuterium oxide washout. Actical activity monitors were placed on each cat's collar to allow quantification of the activity of each cat. Circulating serum T3 and T4 was measured on days 0, 14, and 28. On day 28, a thyrotropin‐releasing hormone (TRH) stimulation test was conducted to determine treatment effects on serum concentrations of thyroid hormones. There was a significant increase in daily water consumption with dietary water treatment (192 ml ± 7.85 SEM) compared to the control (120 ml ± 20.4) and selenium (116 ml ± 14.6) treatments. Both water and selenium treatments were associated with greater (p < .05) activity over that of the control treatment by 20.5% and 11% respectively. Serum TT3 AUC concentrations (0–4 hr) of TRH stimulation tests were greater (p < .05) by 16% with water compared to control treatments. The results of this study indicate that dietary water content may alter the function of the thyroid axis and that this effect is associated with an increase in physical activity.     

Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002)

OBJECTIVE: To assess whether the risk of development of hypothyroidism after treatment with iodine 131 (131I) was associated with the pattern of sodium pertechnetate Tc 99m activity in the thyroid gland detected via scintigraphy before treatment in cats with hyperthyroidism. DESIGN: Retrospective study. ANIMALS: 165 cats. PROCEDURE: Medical records of cats with hyperthyroidism that had been treated with 131I (from 1990 to 2002) and had undergone scintigraphy of the thyroid gland before treatment were reviewed; data regarding signalment, scintigraphic findings (classified as unilateral, bilateral-asymmetric, bilateral-symmetric, or multifocal patterns), serum total thyroxine (T4) concentrations before treatment and prior to hospital discharge, and 131I treatment were collected. A questionnaire was sent to each referring veterinarian to obtain additional data including whether the cats subsequently developed hypothyroidism (defined as serum total T4 concentration less than the lower reference limit > or = 3 months after treatment). RESULTS: 50 of 165 (30.3%) 131I-treated cats developed hypothyroidism. Hypothyroidism developed in 39 of 109 cats with bilateral, 10 of 50 cats with unilateral, and 1 of 6 cats with multifocal scintigraphic patterns of their thyroid glands. Cats with a bilateral scintigraphic pattern were approximately 2 times as likely to develop hypothyroidism after 131I treatment than were cats with a unilateral scintigraphic pattern (hazard ratio, 2.1; 95% confidence interval, 1.04 to 4.2). CONCLUSIONS AND CLINICAL RELEVANCE: Cats with hyperthyroidism that have a bilateral scintigraphic pattern in the thyroid gland before 131I treatment appear to have a significantly higher risk of subsequently developing hypothyroidism, compared with cats with a unilateral scintigraphic pattern.     

Serum fructosamine concentration in cats with overt hyperthyroidism.

OBJECTIVE: To determine the effect of hyperthyroidism on serum fructosamine concentration in cats. DESIGN: Cohort study. ANIMALS: 22 cats with overt hyperthyroidism. PROCEDURE: Hyperthyroidism was diagnosed on the basis of clinical signs, detection of a palpable thyroid gland, and high total serum thyroxine (T4) concentrations. Hyperthyroid cats with abnormal serum albumin, total protein, and glucose concentrations were excluded from the study. Samples for determination of serum fructosamine concentration were obtained prior to initiating treatment. Results were compared with fructosamine concentrations in healthy cats, cats in which diabetes had recently been diagnosed, and cats with hypoproteinemia. In 6 cats, follow-up measurements were obtained 2 and 6 weeks after initiating treatment with carbimazole. RESULTS: Serum fructosamine concentrations ranged from 154 to 267 mumol/L (median, 198 mumol/L) and were significantly lower than values in healthy cats. Eleven (50%) of the hyperthyroid cats had serum fructosamine concentrations less than the reference range. Serum fructosamine concentrations in hyperthyroid, normoproteinemic cats did not differ from values in hypoproteinemic cats. During treatment, an increase in serum fructosamine concentration was detected. CONCLUSIONS AND CLINICAL RELEVANCE: In hyperthyroid cats, concentration of serum fructosamine may be low because of accelerated protein turnover, independent of blood glucose concentration. Serum fructosamine concentrations should not be evaluated in cats with overt hyperthyroidism and diabetes mellitus. Additionally, concentration of serum fructosamine in hyperthyroid cats should not be used to differentiate between diabetes mellitus and transitory stress-related hyperglycemia.     

ACCURACY OF INCREASED THYROID ACTIVITY DURING PERTECHNETATE SCINTIGRAPHY BY SUBCUTANEOUS INJECTION FOR DIAGNOSING HYPERTHYROIDISM IN CATS

Our purpose was to determine the accuracy of increased thyroid activity for diagnosing hyperthyroidism in cats suspected of having that disease during pertechnetate scintigraphy using subcutaneous rather than intravenous radioisotope administration. Increased thyroid activity was determined by two methods: the thyroid:salivary ratio (T:S) and visual inspection. These assessments were made on the ventral scintigram of the head and neck. Scintigraphy was performed by injecting sodium pertechnetate (111 MBq, SQ) in the right-dorsal-lumbar region; static-acquisition images were obtained 20 min after injection. We used 49 cats; 34 (69%) had hyperthyroidism based on serum-chemistry analysis. Using a Wilcoxon's rank-sum test, a significant difference (P < 0.0001) was detected in the T:S between cats with and without hyperthyroidism. Using a decision criterion of 2.0 for the T:S, the test accurately predicted hyperthyroidism in 32/34 cats (sensitivity, 94%; 95% confidence interval (CI), 85-100%) and correctly predicted that hyperthyroidism was absent in 15/15 cats (specificity, 100%; CI, 97-100%). Using visual inspection, the test accurately predicted hyperthyroidism in 34/34 cats (sensitivity, 100%; CI, 99-100%) and correctly predicted that hyperthyroidism was absent in 12/15 cats (specificity, 80%; CI, 56-100%). The positive and negative predictive values were high for a wide range of prevalence of hyperthyroidism. And, the test had excellent agreement within and between examiners. Therefore, detecting increased thyroid activity during pertechnetate scintigraphy by subcutaneous injection is an accurate and reproducible test for feline hyperthyroidism.     

Use of the triiodothyronine suppression test for diagnosis of hyperthyroidism in ill cats that have serum concentration of iodothyronines within normal range.

Administration of triiodothyronine (liothyronine, 15 micrograms, q 8 h, for 6 treatments) caused marked decrease in serum concentration of thyroxine (T4) and estimates of free T4 (fT4) concentration in clinically normal cats. A prospective clinical study was done to evaluate the use of this suppression test for diagnosis of hyperthyroidism in cats with clinical signs suggestive of the disease, but lacking high serum concentration of iodothyronines. Twenty-three cats were confirmed as hyperthyroid on the basis of histologic changes in the thyroid gland or clinical improvement in response to administration of methimazole. Mean +/- SD serum concentration of T4 (34.3 +/- 12.7 to 31.3 +/- 11.5 nmol/L) and estimate of fT4 concentration (26.6 +/- 6.4 to 25.6 +/- 6.9 pmol/L) did not change after administration of liothyronine to these cats. Twenty-three cats were classified as nonhyperthyroid by histologic confirmation of other disease, abnormal results of other diagnostic tests that strongly supported primary disease other than hyperthyroidism, or spontaneous remission of weight loss without treatment. Mean +/- SD serum concentration of T4 (27.9 +/- 10.3 to 11.7 +/- 6.4 nmol/L) and estimate of fT4 concentration (21.7 +/- 5.4 to 10.4 +/- 4.4 pmol/L) decreased significantly (P less than 0.001) in response to administration of liothyronine. Discriminant analysis was used to identify variables from iodothyronine assays (eg, absolute concentration of T4 or absolute estimate of fT4 concentration, or changes of T4 or fT4 concentration) that provided the best diagnostic sensitivity and specificity.(ABSTRACT TRUNCATED AT 250 WORDS)     

EVALUATION OF QUANTITATIVE THYROID SCINTIGRAPHY FOR DIAGNOSIS AND STAGING OF DISEASE SEVERITY IN CATS WITH HYPERTHYROIDISM: COMPARISON OF THE PERCENT THYROIDAL UPTAKE OF PERTECHNETATE TO THYROID‐TO‐SALIVARY RATIO AND THYROID‐TO‐BACKGROUND RATIOS

Thyroid scintigraphy is commonly used for evaluation of cats with hyperthyroidism, with the thyroid‐to‐salivary ratio (T/S) being the most common method to quantify the degree of thyroid activity and disease. Calculation of thyroid‐to‐background ratios (T/B) or percent thyroidal uptake of ^99mTcO^?₄ (TcTU) has only been reported in a few studies. The purpose of this prospective, cross‐sectional study was to evaluate a number of quantitative scintigraphic indices as diagnostic tests for hyperthyroidism, including the T/S, three different T/B, TcTU, and estimated thyroid volume. Of 524 cats referred to our clinic for evaluation of suspected hyperthyroidism, the diagnosis was confirmed (n = 504) or excluded (n = 20) based on results of a serum thyroid panel consisting of thyroxine (T₄), triiodothyronine (T₃), free T₄ (fT₄), and thyroid‐stimulating hormone (TSH) concentrations. In the hyperthyroid cats, median values for TcTU, T/S, and three T/B ratios were all significantly higher (P < 0.001) than values in euthyroid suspect cats or clinically normal cats. All scintigraphic parameters were relatively sensitive and specific as diagnostic tests for hyperthyroidism, but the T/S ratio had the highest test accuracy. The T/S ratio correlated strongly with the TcTU (r = 0.85). However, the TcTU had a higher and more significant correlation (P < 0.01) with serum T₄ (r = 0.76 vs. 0.64), T₃ (r = 0.77 vs. 0.64), and estimated thyroid volume (r = 0.62 vs. 0.38). Overall, calculation of TcTU is an accurate diagnostic test, but also appears to be the best parameter to predict the functional volume and metabolic activity of the feline adenomatous thyroid gland.     

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.     

Percutaneous ultrasound-guided radiofrequency heat ablation for treatment of hyperthyroidism in cats

OBJECTIVE: To determine efficacy and safety of percutaneous radiofrequency heat ablation for treatment of hyperthyroidism in cats. DESIGN: Prospective study. ANIMALS: 9 cats. PROCEDURE: Hyperthyroidism was diagnosed via clinical signs and high serum total (TT4) and free thyroxine (fT4) concentrations. One or 2 hyperfunctional cervical thyroid nodules were detected by use of scintigraphy and ultrasonography. If cats had 1 abnormal thyroid lobe, heat ablation was performed on that lobe; if cats had 2 abnormal lobes, heat ablation was applied to the larger lobe. Overall, heat ablation was performed 14 times in the 9 cats. Clinical signs and serum TT4, fT4, and calcium concentrations were monitored daily for 2 days after the procedure, weekly for the first month, and then monthly. Laryngeal function was evaluated and cervical ultrasonography and thyroid scintigraphy were also performed. Monitoring continued for as long as 9 months after heat ablation if a cat became euthyroid or until an owner chose an alternative treatment because of recurrence of hyperthyroidism. RESULTS: Serum TT4 and fT4 concentrations transiently decreased after all 14 heat ablation procedures (< or = reference range after 10 of 14 treatments) within 2 days after the procedure. Cats were euthyroid for 0 to 18 months (mean, 4 months). Hyperthyroidism recurred in all cats. Adverse effects included transient Horner's syndrome (2 cats) and laryngeal paralysis without clinical signs (1 cat). CONCLUSIONS AND CLINICAL RELEVANCE: Percutaneous heat ablation as a treatment for hyperthyroidism in cats is effective transiently but not permanently.     

Serum thyroxine concentrations after radioactive iodine therapy in cats with hyperthyroidism

Thirty-one cats with hyperthyroidism were given one dose of radioactive iodine (131I) IV. Serum thyroxine (T4) concentrations were measured before treatment in all cats, at 12-hour intervals after treatment in 10 cats, and at 48-hour intervals after treatment in 21 cats. Serum T4 concentrations also were measured one month after 131I therapy in 29 cats. Activity of 131I administered was 1.5 to 6.13 mCi, resulting in a dose of 20,000 rads to the thyroid. Serum T4 concentrations before 131I administration were 5.3 to 51.0 micrograms/dl, with a median T4 concentration of 11.0 micrograms/dl. Serum T4 decreased most rapidly during the first 3 to 6 days after treatment. Sixteen cats (55%) had normal serum thyroxine concentrations by day 4 after 131I administration, and 23 cats (74%) were euthyroxinemic by day 8 after treatment. One month after administration of 131I, the 29 cats evaluated were clinically improved, and 24 (83%) of the 29 cats evaluated had normal serum T4 concentrations, 3 cats (10%) remained hyperthyroxinemic, and 2 cats (7%) were hypothyroxinemic. Therefore, administration of 131I was a safe and effective method to quickly decrease serum T4 concentrations in hyperthyroid cats.     

Triiodothyronine (T3) suppression test. An aid in the diagnosis of mild hyperthyroidism in cats

The purpose of this study was to develop a T3 suppression test to help in the diagnosis of mild hyperthyroidism in cats. We evaluated the response in circulating T4 concentrations to exogenous T3 (liothyronine) administration in 44 clinically normal cats, 77 cats with hyperthyroidism, and 22 cats with nonthyroidal disease. The test was performed by first collecting blood samples for basal serum T4 and T3 determinations, administering liothyronine at an oral dosage of 25 micrograms three times daily for seven doses, and, on the morning of the third day, again collecting serum samples for T4 and T3 determinations 2 to 4 hours after the seventh dose of liothyronine. The mean basal serum concentrations of T4 (53.1 nmol/L) and T3 (1.8 nmol/L) were significantly higher in the cats with hyperthyroidism than in the normal cats (T4 = 25.3 nmol/L, T3 = 1.3 nmol/L) and the cats with nonthyroidal disease (T4 = 29.5 nmol/L, T3 = 1.4 nmol/L); however, there was a great deal of overlap of basal values between the three groups of cats. Of the 77 cats with mild hyperthyroidism, 41 (53%) had serum T4 values and 55 (71%) had T3 values that were within the established normal ranges. After administration of liothyronine, mean serum T4 concentrations fell much more markedly in the normal cats and the cats with nonthyroidal disease than in the hyperthyroid cats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyroid function and pregnancy status in broodmares

OBJECTIVE: To determine whether thyroid function was associated with pregnancy status in broodmares. DESIGN: Prospective study. ANIMALS: 79 Thoroughbred and Standardbred broodmares between 2 and 22 years old. PROCEDURE: Serum triiodothyronine (T3) concentration was measured before and 2 hours after i.v. administration of thyrotropin releasing hormone (TRH), and serum thyroxine (T4) concentration was measured before and 4 hours after TRH administration. Pregnancy status was monitored by means of transrectal ultrasonography beginning 16 days after ovulation. RESULTS: Baseline T3 and T4 concentrations varied widely. In all mares, serumT3 concentration increased in response to TRH administration. Serum T4 concentration increased in response toTRH administration in all but 2 mares. Pregnancy rate was 76%. Baseline and stimulated serum T3 and T4 concentrations were not significantly different between mares that became pregnant and those that did not. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that decreased thyroid function is uncommon in mares and poor thyroid function is not a common cause of infertility. Thus, the practice of indiscriminately treating broodmares with thyroid hormone to enhance fertility appears questionable at this time.     

ULTRASONOGRAPHIC EXAMINATION OF THE THYROID GLAND OF HYPERTHYROID CATS: COMPARISON TO 99mTcO−4 SCINTIGRAPHY

High-resolution ultrasonography was evaluated as an alternative to ^99mTcO^-₄ scintigraphy for examining size and appearance of thyroid glands in hyperthyroid cats. Thyroid ultrasound examinations were performed on 6 normal cats and 14 cats with hyperthyroidism. Thyroid lobe volume was estimated from ultrasound images using the equation for a prolate ellipsoid, π/6 (length * height * width). Total thyroid volume was estimated by adding the volume estimations of the left and right lobes. Thyroid lobes of hyperthyroid cats were considered abnormal if estimated volume exceeded the 99% confidence interval for normal thyroid volume determined from the control group. Scintigraphic examinations performed on hyperthyroid cats were evaluated for unilateral versus bilateral disease and for the presence of ectopic activity. Mean thyroid lobe volume and total thyroid volume for normal cats was 85 and 169 mm³, respectively. Mean thyroid lobe volume and total thyroid volume for hyperthyroid cats was 578 and 889 mm³. There was a significant difference in mean estimated total thyroid volume of normal and hyperthyroid cats. Thyroid lobes with greater than normal TcO^-₄ uptake on scintigraphy were larger and had variable homogeneity, echogenicity, and margination on ultrasound examination. There also was an 85.7% agreement of scintigraphy and ultrasonography in differentiating normal from abnormal thyroid lobes. A fair correlation between estimated total thyroid volume of hyperthyroid cats and most recent pretherapy serum thyroxine values were also found. This preliminary study indicates that thyroid ultrasound examination may provide information that is useful for diagnosis and treatment of feline hyperthyroidism. Although ultrasound provides accurate evaluation of the thyroid glands, it cannot replace ^99mTcO^-₄ scintigraphy for screening of metastatic lesions and ectopic glands.     

Effects of thyrotropin-releasing hormone on serum concentrations of thyroxine and triiodothyronine in healthy, thyroidectomized, thyroxine-treated, and propylthiouracil-treated dogs

Effects of thyrotropin-releasing hormone (TRH) on serum concentrations of thyroid hormones were studied in 36 mixed-bred dogs. Dogs were randomly assigned to 7 groups. Significant increases (P less than 0.05) of serum thyroxine (T4) values occurred as early as 2 hours and reached a peak at 6 to 8 hours after IV injection of 300 to 1,100 micrograms of TRH. Thyroxine concentrations in response to a TRH dose greater than 500 micrograms were similar to those observed with the 300-micrograms dose. Transient coughing, vomiting, salivation, and defecation after large doses (900 and 1,100 micrograms) were observed. Mean serum T4 concentration decreased from 2.1 micrograms/dl to 0.9 micrograms/dl within 1 day of thyroidectomy. Clinical signs of hypothyroidism, including lethargy, dry coats, and diffuse alopecia, were present in 2 dogs at a month after surgical operation. Thyroxine concentrations were detectable for greater than 2 months. Injection (IV) of 700 micrograms of TRH 6 weeks after surgical operation had no effect on serum concentration of T4 in thyroidectomized dogs. In 5 T4-treated dogs, TRH (700 micrograms, IV) significantly increased the serum T4 value, indicating that pituitary thyrotropes were responsive to TRH, in spite of daily medication of 0.8 mg of T4. Four dogs were treated orally with 200 mg of propylthiouracil/day for 5 weeks. Intravenous injection of 700 micrograms of TRH in propylthiouracil-treated dogs had no effect on the serum T4 concentration, indicating that TRH had no effect on serum T4 values in these dogs during the experimental period. These results indicate that TRH can replace bovine thyrotropin for the canine thyroid function test.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of relationships between pretreatment patient variables and duration of isolation for radioiodine-treated hyperthyroid cats

OBJECTIVE: To determine relationships between commonly measured pretreatment variables and duration of isolation for unrestricted dismissal after oral administration of iodine 131 (131I) for treatment of hyperthyroidism in cats. ANIMALS: 149 hyperthyroid cats treated with 131I. PROCEDURE: A dose of 131I (2.9 to 6.04 mCi [1.07 to 2.23 x 10(8) Bq]) was administered orally to all cats after hyperthyroidism was confirmed by evaluation of serum total thyroxine (T4) concentrations. Forward stepwise regression analysis was used to determine whether pretreatment total T4 concentration, serum creatinine concentration, body weight, age, 131I dose, or concurrent administration of cardiac medication (specifically excluding thyroid suppression drugs) could be used as pretreatment predictors of duration of isolation in a clinical setting. Gamma radiation emission rate at dismissal was < 2.0 mR/h at skin surface over the thyroid region. RESULTS: Mean +/- SD duration of isolation was 16.67 +/- 4.42 days (95% confidence interval, 9.2 to 24.1 days). The regression equation for duration of isolation calculated on the basis of dose of 131I (duration of isolation [days] = 3.2 + [2.66 X mCi - 131I dose]) yielded a regression line with a 95% confidence interval of +/- 3.3 days; only 15% of the variation was explained. CONCLUSIONS AND CLINICAL RELEVANCE: A pretreatment estimate for duration of isolation could be determined only from an equation based on the orally administered dose of 131I. These findings suggest that administration of the lowest efficacious dose possible is the dominant factor in reduction of duration of isolation for cats treated with 131I.     

Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats

OBJECTIVE: To compare effects of short-term administration of a soy diet with those of a soy-free diet on serum thyroid hormone concentrations in healthy adult cats. ANIMALS: 18 healthy adult cats. PROCEDURE: Cats were randomly assigned to receive either a soy or soy-free diet for 3 months each in a crossover design. Assays included CBC, serum biochemical profile, thyroid hormone analysis, and measurement of urinary isoflavone concentrations. RESULTS: Genistein, a major soy isoflavone, was identified in the urine of 10 of 18 cats prior to dietary intervention. Compared with the soy-free diet, cats that received the soy diet had significantly higher total thyroxine (T4) and free T4 (fT4) concentrations, but unchanged total triiodothyronine (T3) concentrations. The T3/fT4 ratio was also significantly lower in cats that received the soy diet. Although the magnitudes of the increases were small (8% for T4 and 14% for fT4), these changes resulted in an increased proportion of cats (from 1/18 to 4/18) that had fT4 values greater than the upper limit of the laboratory reference range. There was no significant effect of diet on any other measured parameter. CONCLUSIONS AND CLINICAL RELEVANCE: Short-term administration of dietary soy has a measurable although modest effect on thyroid hormone homeostasis in cats. Increase in T4 concentration relative to T3 concentration may result from inhibition of 5'-iodothyronine deiodinase or enhanced T3 clearance. Soy is a common dietary component that increases serum T4 concentration in cats.