RADIONUCLIDE THYROID IMAGING IN 135 CATS WITH HYPERTHYROIDISM*

Thyroid scanning was performed in 135 hyperthyroid cats and 13 normal cats with technetium-99m as pertechnetate (^99mTcO₄) or with radioactive iodine (¹³¹I). Of the hyperthyroid cats, enlargement and increased radionuclide accumulation were found in one thyroid lobe in 38 (27%) and in both lobes in 97 (73%). In two hyperthyroid cats with thyroid carcinoma, extension of tumor into the thoracic cavity was detected. In normal and hyperthyroid cats the radionuclide images produced with ^99mTcO₄ and ¹³¹I were similar; however, the quality of the ^99mTcO₄ scans was usually better than that of the ¹³¹I scans.     

PRE‐ AND POSTTREATMENT ULTRASONOGRAPHY OF THE THYROID GLAND IN HYPERTHYROID CATS

Ultrasonography is useful for assessing the morphology of the thyroid gland in hyperthyroid cats. Our aim was to describe the ultrasonographic changes of the thyroid gland in hyperthyroid cats after ¹³¹I therapy. Ultrasonography was performed in 15 hyperthyroid cats at initial presentation and 6 months after ¹³¹I using a multifrequency linear transducer set at 12 MHz. The following criteria were evaluated: length, width, height, volume, shape, homogeneity, and vascularity, using Power Doppler. Pretreatment, 10 cats had bilaterally abnormal thyroid lobes, four cats one abnormal lobe with the contralateral lobe being normal or reduced in size, and one cat with one normal lobe and one lobe not visible. Six months after ¹³¹I therapy, there was a reduction in median volume from 819 to 210 mm³, reduced rounding, reduced heterogeneity, and decreased vascularity. In conclusion, ultrasonography may be used to monitor thyroid changes in order to assess ¹³¹I treatment response. Further studies are necessary to determine whether ultrasonography could contribute to the detection of a relapsing course of hyperthyroidism.     

THE EFFECTS OF IOHEXOL ADMINISTRATION ON TECHNETIUM THYROID SCINTIGRAPHY IN NORMAL CATS

Administration of iodinated contrast medium interferes with iodide uptake in the human thyroid gland and compromises diagnostic thyroid scintigraphy and radioiodine treatment for 4–6 weeks. However, the degree and duration of inhibition of thyroid uptake of pertechnetate (^99mTcO₄^?) by iodinated contrast medium has not been established in any species. The main objective of this study was to better understand the temporal characteristics and magnitude of inhibition of feline thyroid uptake of ^99mTcO₄^? due to iohexol administration. Routine thyroid scintigraphy was performed in eight cats by intravenous (IV) injection of 185 MBq (5 mCi) of ^99mTcO₄^? both 4 days before and 0,1, 3, 7,14, and 28 days after IV administration of 880 mg I/kg iohexol (240 mg I/ml). Thyroid scintigraphy data were used to calculate thyroid:salivary gland ratios (T:S) and the percentage of total injected ^99mTcO₄^? dose uptake within the thyroid (%TU) at 20 min postinjection. After iohexol administration, mean T:S was significantly decreased below baseline only on day 1. At no point during the study did any cat have a T:S that fell below the published normal reference range of 0.71±0.14. There was a significant decrease in %TU on day 1, 3, and 14; however, at no point during the study, did any cat have a %TU that fell below the published normal reference ranges of 0.64±0.57, 0.68±0.9, or 0.75±1.38.     

COMPUTED TOMOGRAPHIC CHARACTERISTICS OF THE THYROID GLANDS IN EIGHT HYPERTHYROID CATS PRE‐ AND POSTMETHIMAZOLE TREATMENT COMPARED WITH SEVEN EUTHYROID CATS

Hyperthyroidism is the most common feline endocrinopathy; thyroid computed tomography (CT) may improve disease detection and methimazole dose selection. Objectives of this experimental pre‐post with historical case‐control study were to perform thyroid CT imaging in awake or mildly sedated hyperthyroid cats, compare thyroid gland CT appearance in euthyroid and hyperthyroid cats pre‐ and postmethimazole treatment, and determine whether thyroid size or attenuation correlate with methimazole dose needed for euthyroidism. Premethimazole treatment, eight hyperthyroid cats received CT scans from the head to heart, which were compared to CT of seven euthyroid cats. Total thyroxine levels were monitored every 3–4 weeks. Postmethimazole CT was performed 30 days after achieving euthyroid status. Computed tomography parameters recorded included thyroid length, width, height, attenuation, and heterogeneity. Median time between CT was 70 days (53–213 days). Mild sedation was needed in five hyperthyroid cats premethimazole, and none postmethimazole. Thyroid volume was significantly larger in hyperthyroid cats compared to euthyroid cats (785.0 mm³ vs. 154.9 mm³; P = 0.002) and remained unchanged by methimazole treatment (?4.5 mm3; P = 0.50). Thyroid attenuation and heterogeneity decreased with methimazole treatment (96.1 HU vs. 85.9 HU; P = 0.02. 12.4 HU vs. 8.1 HU; P = 0.009). Methimazole dose ranged from 2.5 to 10 mg daily with a positive correlation between pretreatment thyroid gland volume and dose needed to achieve euthyroidism (P = 0.03). Euthyroid and hyperthyroid cats are easily imaged awake or mildly sedated with CT. Methimazole in hyperthyroid cats significantly lowers thyroid attenuation and heterogeneity, but not size.     

THE NORMAL FELINE THYROID*

A technique for performing thyroid scintigraphy in the cat using technetium 99m pertechnetate is presented. Two groups of cats were studied: group I—five young adult cats, and group II—five cats, nine to 11 years of age. The resultant scintigrams were uniform in appearance with no significant differences between groups I and II. Computer analysis was performed to determine radioactivity ratios, comparing thyroid with salivary gland and background radioactivity. A consistent thyroid/salivary ratio (T/S) of approximately 1 was obtained for group I and group II. Thyroid/background ratio (T/B) was variable.     

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.     

COMPARISON BETWEEN COMPUTED TOMOGRAPHY AND 99m TC‐ PERTECHNETATE SCINTIGRAPHY CHARACTERISTICS OF THE THYROID GLAND IN CATS WITH HYPERTHYROIDISM

Scintigraphy is currently the reference standard for diagnosing feline hyperthyroidism; however, computed tomography (CT) is more widely available in veterinary practice. The purposes of this prospective study were to describe the CT appearance of thyroid glands in cats with hyperthyroidism and compare CT findings with findings from ^99mTc–pertechnetate scintigraphy. Twenty‐five adult hyperthyroid cats were included. Plain CT images were acquired for each cat and the following characteristics recorded for each thyroid lobe: visibility, delineation, position, attenuation, shape, and subjective size. Scintigraphic images were also acquired and the following characteristics recorded: radiopharmaceutical uptake, delineation, ectopic foci, shape, and subjective size. In CT images, thyroid lobes were most commonly found between the second and fourth cervical vertebrae, dorsolateral to the trachea. Affected thyroid lobes (based on scintigraphy reference standard) were most commonly oval and moderately enlarged in CT images. A heterogeneous attenuation pattern (isoattenuating to adjacent soft tissues with hypo‐ and hyperattenuating foci) was most commonly found in affected thyroid lobes. A positive correlation (P < 0.01) was identified between CT and scintigraphy for left‐to‐right thyroid lobe size relationship and subjective size of the larger thyroid lobe. The CT estimated mass was significantly higher (median = 148.8; range = [0;357.6]) for the more active thyroid lobe compared to the less active thyroid lobe (median = 84.6; range = [0;312.3]); (W = 154; P < 0.01). Findings indicated that CT may not reliably differentiate unilateral vs. bilateral hyperthyroidism in cats; however, CT may be a reliable alternative test for correctly identifying the more active thyroid lobe.     

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.     

THYROID TO SALIVARY RATIOS DETERMINED BY TECHNETIUM-99M PERTECHNETATE IMAGING IN THIRTY-TWO EUTHYROID CATS

Thyroid to salivary (TS) ratio is the most commonly used scintigraphic parameter for differentiating euthyroid and hyperthyroid cats. Studies to determine the normal TS ratio have been performed in small cat populations. In this study, the TS ratio was determined in 32 cats between 8 and 13 years of age. The study population was documented to be euthyroid based on normal initial and 6-week follow-up serum thyroid concentrations and normal T3 suppression tests. All images were obtained with a low-energy all-purpose collimator between 20 and 40 min after the injection of approximately 111 MBq (3.0 mCi) pertechnetate. Manual regions of interest (ROI) were made of the thyroid and salivary glands of the ventral image A 95% prediction interval based on the natural log of the TS ratio was computed to provide a normal range of 0.48-1.66. This range is similar to previous studies, but suggests a slightly higher upper limit than previously reported.     

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.     

Relationship between orally administered dose, surface emission rate for gamma radiation, and urine radioactivity in radioiodine-treated hyperthyroid cats

OBJECTIVE: To determine the relationship between surface emission rate of gamma radiation and urine concentration of I131 (urine radioactivity) during the period 7 to 21 days after oral or SC administration of I131 to hyperthyroid cats. ANIMALS: 47 hyperthyroid cats administered I131 PO and 24 hyperthyroid cats administered I131 SC. PROCEDURE: A dose of I131 (1.78 to 2.04 X 10(2) MBq [4.8 to 5.5 mCi]) was administered orally. Surface emission at the skin adjacent to the thyroid gland on days 7, 10, 14, 18, and 21 and number of counts/30 s in a urine sample (1 mL, obtained via cystocentesis) on days 7, 14, and 21 after oral administration were measured. Effective half-life (T1/2E) was derived for each point. Surface emission thresholds for maximum urine radioactivity values were established. A dose of I131 (1.48 X 10(2) MBq [4.0 mCi]) was administered SC. Urine radioactivity and surface emission rates for SC administration were compared with values for oral administration. RESULTS: The T1/2E for surface emissions and urine radioactivity progressively increased toward values for physical T1/2 over time. The T1/2E for surface emissions was 2.19 to 4.70 days, and T1/2E for urine radioactivity was 2.16 to 3.67 days. Surface emission rates had a clinically useful threshold relationship to maximum urine concentrations of I131. CONCLUSIONS AND CLINICAL RELEVANCE: Surface emission rates for cats administered I131 appeared useful in determining upper limits (threshold) of urine radioactivity and are a valid method to assess the time at which cats can be discharged after I131 administration.     

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.     

THYROID SCINTIGRAPHY FINDINGS IN 2096 CATS WITH HYPERTHYROIDISM

Thyroid scintigraphy is currently the reference standard for diagnosing and staging cats with hyperthyroidism, but few studies describing the scintigraphic characteristics in a large number of cats have been reported. The objective of this study was to better characterize thyroid scintigraphy findings by evaluating 2096 consecutive cats with hyperthyroidism that were referred over a 3.5‐year period. Of these cats, 2068 (98.7%) had a high thyroid‐to‐salivary ratio (>1.5), whereas 2014 (96.1%) were found to have a high thyroid‐to‐background ratio (>6.1). When the patterns of the cats’ thyroid disease were recorded, 665 (31.7%) had unilateral disease, 1060 (50.6%) had bilateral‐asymmetric disease (two thyroid lobes unequal in size), 257 (12.3%) had bilateral‐symmetric disease (both lobes similar in size), and 81 (3.9%) had multifocal disease (≥3 areas of increased radionuclide uptake). The number of areas of ^99mTcO^?₄ uptake in the 2096 cats ranged from 1 to 6 (median, 2), located in the cervical area in 2057 (98.1%), thoracic inlet in 282 (13.5%), and in the thoracic cavity in 115 (5.5%). Ectopic thyroid tissue (e.g. lingual or mediastinal) was diagnosed in 81 (3.9%) cats, whereas thyroid carcinoma was suspected in 35 (1.7%) of the cats. The results of this study support conclusions that most hyperthyroid cats have unilateral or bilateral thyroid nodules, but that multifocal disease will develop in a few cats that have ectopic thyroid disease or thyroid carcinoma. Both ectopic thyroid disease and thyroid carcinoma are relatively uncommon in hyperthyroid cats, with a respective prevalence of ～4% and ～2% in this study.     

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.     

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.     

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.     

EVALUATION OF THYROID TO BACKGROUND RATIOS AND COMPARISON OF VARIOUS SCINTIGRAPHIC MEASUREMENTS AND THEIR CORRELATION TO SERUM T4 IN HYPERTHYROID CATS

Thyroid‐to‐salivary ratio and percent dose uptake are the most widely recognized scintigraphic measurements. Recently, the thyroid‐to‐background ratio has been proposed as an alternate method. However, this method has not been validated. The purpose of this observational, cross‐sectional, prospective study was to determine the location of a background region of interest (ROI) that is most reflective of blood pool activity. We also hypothesized that the thyroid‐to‐background ratio using this background ROI would be a better predictor of thyroid function. Fifty‐six cats presented to the Virginia‐Maryland College of Veterinary Medicine seeking radioiodine therapy for hyperthyroidism were enrolled in this cross‐sectional study to evaluating thyroid‐to‐background ratio. A blood sample for measuring plasma radioactivity was collected at the time of scintigraphy. The plasma radioactivity was compared to the background ROIs in eight anatomic regions. Scintigraphic measures of thyroid‐to‐background and thyroid‐to‐salivary ratios, and percent dose were then compared to serum T₄. The heart ROI was most closely correlated with plasma pertechnetate activity (r = 0.70). Percent dose uptake was most closely correlated with serum T₄ (r = 0.74), followed by thyroid‐to‐salivary ratio (r = 0.66) and thyroid‐to‐background ratio using the heart ROI (r = 0.59). Thyroid‐to‐background ratio using the heart background ROI is a good predictor T₄ but percent dose uptake and thyroid‐to‐salivary ratio proved to be better predictors of T₄ than any of the thyroid‐to‐background ratios.     

QUANTITATIVE PERTECHNETATE THYROID SCINTIGRAPHY AND THE ULTRASONOGRAPHIC APPEARANCE OF THE THYROID GLAND IN CLINICALLY NORMAL HORSES

We characterized the scintigraphic and sonographic appearance of the thyroid gland in clinically normal horses to establish the value of these modalities for assessment of the thyroid gland in this species. Horses were divided into two age groups. One group consisted of eight horses between 3 and 10 years of age and the other of seven horses between 11 and 20 years of age. Total T₄ concentrations were within the laboratory reference interval in all horses. Thyroid to salivary (T/S) ratio, percent dose uptake of pertechnetate (Na^99mTcO₄) and thyroid lobe volume were calculated. The echogenicity of thyroid lobes and presence of nodules was documented. The two groups were compared using appropriate parametric and nonparametric statistics. Mean total T₄ concentration was lower in older horses. Sixty minute mean±standard deviation (SD) T/S ratios for old vs. young horses were 5.8±3.0 and 5.3±2.2, respectively. Sixty‐minute median and interquartile ranges for percent dose uptake of pertechnetate for old vs. young horses were 3.64% (1.5–3.98%) and 2.55% (2.33–2.90%), respectively. Mean±SD thyroid lobe volume for old vs. young horses were 18.93±5.16 cm ³ and 13.55±3.56 cm³, respectively. Differences between groups were not significant. Most thyroid lobes were hyper or isoechoic to the sternocephalicus muscle. Prevalence of thyroid nodules did not differ between groups. Further study is needed to determine if thyroidal percent dose uptake is significantly different in horses with thyroid dysfunction and if it is clinically useful.     

QUALITATIVE AND QUANTITATIVE THYROID IMAGING IN FELINE HYPERTHYROIDISM USING TECHNETIUM-99M AS PERTECHNETATE

Thyroid imaging using technetium-99m as pertechnetate (^99mTcO₄) was carried out in five healthy, euthyroid and 37 hyperthyroid cats using both pinhole and parallel-hole collimators. Images of greater resolution, necessary to distinguish bilateral lobe involvement, were obtained using the pinhole collimator. Per cent thyriod ^99mTcO₄ - uptake was calculated in each cat and was significanly (P < 0.001) higher in hyperthyroid compared with euthyroid cats. In the hyperthyroid cats, per cent thyroid uptake was significantly correlated with serum total thyroxine (T4) and triiodothyronine (T3) Concentrations. Per cent thyroid ^99mTcO₄ - uptake is increased in feline hyperthyrodism and may be calculated using a pinhole collimator alone at the time of qalitative assessment of the extent of thyroid tissue involvement.     

Serum thyroxine concentrations following fixed-dose radioactive iodine treatment in hyperthyroid cats: 62 cases (1986-1989)

The medical records of 62 hyperthyroid cats treated with a fixed dose of 4 mCi of radioactive iodine (131I) were reviewed. In 60 cats, serum thyroxine concentrations were determined after treatment, allowing evaluation of treatment success. Eighty-four percent of the cats had normal serum thyroxine concentrations after treatment. Five of the 60 cats (8%) remained hyperthyroxinemic after treatment. Five cats (8%) were hypothyroxinemic when evaluated within 60 days of treatment. Three of these cats had normal serum thyroxine concentrations 6 months after treatment, and none had clinical signs of hypothyroidism. The administration of a fixed dose of 4 mCi of 131I was determined to be an effective treatment for feline hyperthyroidism.