Recombinant human thyrotropin administration enhances thyroid uptake of radioactive iodine in hyperthyroid cats

BACKGROUND: Hyperthyroidism is the most diagnosed endocrine disorder in cats and radioiodine (131I) is the treatment of choice. The dose emission rate and radioactivity in urine, saliva, and on hair and paws are determined by the dose of administered 131I. A dose reduction of therapeutic 131I could possibly be achieved after recombinant human thyrotropin (rhTSH) administration as in humans with nodular goiter. HYPOTHESIS: rhTSH will increase radioiodine uptake in hyperthyroid cats. ANIMALS: Five hyperthyroid cats. METHODS: Twenty-five micrograms rhTSH (day 1) or 2 mL 0.9% sodium chloride (NaCl) (day 9) was injected IV. One hour later, 11.4 +/- 4.1 (mean +/- SD) MBq 123I was injected IV. Radioactive iodine uptake (RAIU) was measured 6, 12, and 24 hours after rhTSH (RAIU-rhTSH) or NaCl (RAIU-blanco) injection. Blood samples for measurement of TT4 were taken before injection of rhTSH or NaCl (TT4(0)) and at the time of imaging. RESULTS: Percentages of RAIU-rhTSH (and RAIU-blanco) at 6, 12, and 24 hours after administration of rhTSH were 34 +/- 18 (31 +/- 21), 46 +/- 20 (38 +/- 18), and 47 +/- 15 (36 +/- 14). There was a statistically significant effect of rhTSH administration on RAIU (P = .043) but not on serum TT4 concentration. Baseline serum TT4(0) concentration influenced RAIU-rhTSH significantly at 6 hours (P = .037). CONCLUSION AND CLINICAL IMPORTANCE: The increased RAIU observed after rhTSH administration in hyperthyroid cats could lead to a lower therapeutic dose of 131I after rhTSH administration in hyperthyroid cats and decreased risk of environmental and owner contamination during and after hospitalization.     

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.     

QUANTITATIVE THYROID SCINTIGRAPHY AS A PREDICTOR OF SERUM THYROXIN CONCENTRATION IN NORMAL AND HYPERTHYROID CATS

Quantitative thyroid scintigraphy using pertechnetate was performed in 43 cats with various T4 concentrations and compared to eight normal control cats. Quantitative parameters included percentage dose uptake of the radioisotope by the thyroid, thyroid:salivary ratio and rate of thyroid uptake. All cats were anesthetized for the scan, and images were obtained using both low-energy all purpose (LEAP) and pinhole collimators. All quantitative parameters were significantly correlated to the serum T4 concentration, but the best correlation was obtained using the 20-minute thyroid:salivary ratio using only the most intense of the two thyroid lobes. The thyroid:salivary ratio was a good predictor of the metabolic status of the thyroid.     

Interference of iohexol with radioiodine thyroid uptake in the hyperthyroid cat

Absorbed thyroid dose and effective half-life were determined in 46 hyperthyroid cats after treatment with a low dose (mean 111MBq) of radioiodine intravenously. Thirteen of these cats had received iohexol for glomerular filtration rate (GFR) measurement within 24h before treatment with radioiodine in view of another ongoing study at our institution. Pre-therapy values were obtained for total thyroxine (TT(4)) and for the thyroid to salivary gland ratio with sodium pertechnetate gamma-camera imaging. All cats underwent post-therapy scans at 24, 48 and 120 h for evaluation of radioactive iodine uptake (RAIU) and the effective half-life of radioiodine. The absorbed dose was calculated from the cumulative activity with Olinda software. Both groups were comparable in age, TT(4) and the ratio of thyroid activity to salivary gland activity. Statistical analysis revealed a significant decreased absorbed dose in the thyroid in the iohexol group. This decreased uptake was not accompanied by an decreased effective half-life of the radioiodine. The variation of inter-individual RAIU decreased in this group and more homogenous absorbed doses were obtained. No significant difference in outcome could be demonstrated. However, a tendency towards a higher number of residual hyperthyroidism in the iohexol group was noted (15 versus 6% in control group). This study demonstrates that iohexol interferes with the uptake of radioiodine in the hyperthyroid cat but does not provoke increased turnover. In this study, albeit including a small number of cats, outcome did not seem to be significantly affected.     

Use of recombinant human thyroid-stimulating hormone for thyrotropin-stimulation testing of euthyroid cats

OBJECTIVE: To evaluate response of euthyroid cats to administration of recombinant human thyroid-stimulating hormone (rhTSH). ANIMALS: 7 healthy cats. PROCEDURE: Each cat received each of 5 doses of rhTSH (0, 0.025, 0.050, 0.100, and 0.200 mg), IV, at 1-week intervals. Serum concentration of total thyroxine (TT4) and free thyroxine (fT4) was measured immediately before each injection (time 0) and 2, 4, 6, and 8 hours after administration of each dose. RESULTS: Overall TT4 response did not differ significantly among cats when administered doses were > or = 0.025 mg. Serum TT4 concentrations peaked 6 to 8 hours after administration for all doses > or = 0.025 mg. For all doses > or = 0.025 mg, mean +/- SEM TT4 concentration at 0, 6, and 8 hours was 33.9 +/- 1.7, 101.8 +/- 5.9, and 101.5 +/- 5.7 nmol/L, respectively. For all doses > or = 0.025 mg, mean fT4 concentration at 0, 6, and 8 hours was 38.7 +/- 2.9, 104.5 +/- 7.6, and 100.4 +/- 8.0 pmol/L, respectively. At 8 hours, the fT4 response to 0.025 and 0.050 mg was less than the response to 0.100 and 0.200 mg. Adverse reactions after rhTSH administration were not detected. CONCLUSIONS AND CLINICAL RELEVANCE: The TSH stimulation test can be performed in cats by IV administration of 0.025 to 0.200 mg of rhTSH and measurement of serum TT4 concentrations at time of injection and 6 or 8 hours later. Clinical validation of the TSH stimulation test would facilitate development of additional tests of thyroid gland function, such as a TSH assay.     

Effect of four sedative and anesthetic protocols on quantitative thyroid scintigraphy in euthyroid cats

OBJECTIVES: To determine the effect of sedation and anesthesia on thyroid and salivary gland uptake of technetium Tc 99m pertechnetate ((99m)TcO(4)) in euthyroid cats. ANIMALS: 6 euthyroid cats. PROCEDURES: Thyroid scintigraphy was performed by use of a high-resolution low-energy parallel-hole collimator after IV injection of 117 to 133 MBq (3.16 to 3.59 mCi) of (99m)TcO(4)(-). The procedure was performed 4 times on each cat during different sedative and anesthetic protocols in a rotating schedule as follows: propofol, ketamine-midazolam-atropine, ketaminemidazolam, and medetomidine. Regions of interest were drawn around thyroid and salivary glands and counts corrected for background and decay. Percentage of (99m)TcO(4)(-) uptake in salivary and thyroid glands and thyroid-to-salivary gland (99m)TcO(4)(-) uptake ratio were calculated at 20 and 40 minutes. Relative effects of anesthesia and sedation on salivary and thyroid gland (99m)TcO(4)(-) uptake were compared. RESULTS: Significant differences among sedativeanesthetic protocols were found for thyroid gland (99m)TcO(4)(-) uptake, salivary gland (99m)TcO(4)(-) uptake, and thyroid-to-salivary gland (99m)TcO(4)(-) uptake ratio. Thyroid gland (99m)TcO(4)(-) uptake for the ketamine-midazolam protocol at 20 and 40 minutes after (99m)TcO(4)(-) administration was significantly higher than for the propofol protocol. A significant difference in salivary gland(99m) TcO(4)(-) uptake was found between ketamine-midazolam and ketamine-midazolam-atropine protocols at 40 minutes. The thyroid-to-salivary gland (99m)TcO(4)(-) uptake ratio for the ketamine-midazolam protocol was significantly higher at 40 minutes than for propofol or ketamine-midazolam-atropine protocols. CONCLUSIONS AND CLINICAL RELEVANCE: Sedation and anesthesia have a significant effect on thyroid and salivary gland (99m)TcO(4) uptake in euthyroid cats that may interfere with thyroid scintigraphic image interpretation.     

CALCULATION AND USAGE OF THE THYROID TO BACKGROUND RATIO ON THE PERTECHNETATE THYROID SCAN

Feline hyperthyroidism is a common endocrine disorder. A single dose of 148 MBq (4 mCi) ¹³¹I is 95–98% effective for the treatment of hyperthyroidism in cats; however, the cause for treatment failures has not been determined. In a series of 113 hyperthyroid cats having pertechnetate thyroid scintigraphy before treatment using a standard 148 MBq (4 mCi) ¹³¹I dose, the thyroid to salivary gland (T:S) ratio and the thyroid to background (T:B) ratio were calculated. Results in 107 (95%) cats successfully treated were compared with results in six (5%) cats that remained hyperthyroid after treatment. T:B ratio was significantly higher for cats that had treatment failure (median 13.0, range 3.6–73.0) than for cats successfully treated (median 4.4, range 1.2–69.0) (P=0.02), whereas there was no significant difference in their T:S ratios (P=0.2). The T:B ratio is a new approach to evaluating the thyroid pertechnetate scan with the intent of identifying which hyperthyroid cats may fail treatment using a standard 148 MBq (4 mCi) ¹³¹I dose and which, therefore, require a higher dose.     

Effect of recombinant human TSH on the uptake of radioactive iodine (I) by the thyroid gland in healthy beagles

In human medicine, recombinant human thyroid-stimulating hormone (rhTSH) increases thyroid radioactive iodine uptake (RAIU), allowing radioiodine-131 (¹³¹I) dose reduction and greater efficacy in the treatment of differentiated thyroid cancer and multinodular goiter. The goal of this study was to evaluate the effect of rhTSH, administered 24 h and 48 h before radioiodine-123 (¹²³I), on the thyroid RAIU in healthy dogs. Seven healthy euthyroid beagles were randomly allocated to 3 groups (2 groups of 2 dogs and 1 group of 3 dogs) in a prospective, blinded, crossover study. At Week 1, 1 group received ¹²³I for a baseline RAIU; 1 group received 100 μg of rhTSH IV 24 h before ¹²³I, and 1 group received 100 μg of rhTSH IV 48 h before ¹²³I. All dogs received 37 MBq of radioactive ¹²³I IV, and thyroid RAIU was determined 8 h, 24 h, and 48 h thereafter. The study was designed in such a manner that each dog received the 3 treatments and a wash-out period of 3 wk was respected in between. Blood samples were taken for measurement of serum total thyroxine (TT4) and thyrotropin (TSH) concentrations at baseline and 6 h, 12 h, 24 h, and 48 h after rhTSH administration. Recombinant human TSH caused no significant change on thyroid RAIU. The overall mean thyroid RAIU significantly decreased during the study independent of the treatment. Recombinant human TSH significantly increased serum TT4 concentration, which peaked 6 h after rhTSH administration. Compared to baseline, serum TSH concentration remained higher at 6 h, 12 h, 24 h, and 48 h. However, a statistically significant difference was reached only at 6 h and 12 h after rhTSH administration. No adverse effects of rhTSH were observed during the study. Further studies are needed to determine the best timing and dosage of administration of rhTSH in healthy and thyroid carcinoma dogs.     

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.     

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.     

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.     

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.     

Comparison of the biological activity of recombinant human thyroid-stimulating hormone with bovine thyroid-stimulating hormone and evaluation of recombinant human thyroid-stimulating hormone in healthy dogs of different breeds

OBJECTIVE: To evaluate whether use of recombinant human (rh) thyroid-stimulating hormone (TSH) induces equivalent stimulation, compared with bovine TSH (bTSH), and to evaluate activity of rhTSH in dogs of various large breeds. ANIMALS: 18 healthy research Beagles and 20 healthy client-owned dogs of various breeds with body weight > 20 kg. PROCEDURES: The 18 Beagles were randomly assigned to 3 groups, and each dog received either 75 microg of rhTSH, IM or IV, or 1 unit of bTSH, IM, respectively, in a crossover design. The 20 client-owned dogs received 75 microg of rhTSH, IV. Blood samples were taken before and 6 hours after TSH administration for determination of total serum thyroxine (T(4)) concentration. Additional blood samples were taken after 2 and 4 hours in Beagles that received rhTSH, IM. RESULTS: There was a significant increase in T(4) concentration in all dogs, but there were no differences between values obtained after administration of bTSH versus rhTSH or IV versus IM administration of rhTSH. Although there was a significant difference in age and body weight between Beagles and non-Beagles, there was no difference in post-TSH simulation T(4) concentration between the 2 groups. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated an equivalent biological activity of rhTSH, compared with bTSH. Use of 75 microg of rhTSH, IV, did not induce a different magnitude of stimulation in large-breed dogs, compared with Beagles. Euthyroidism was confirmed if post-TSH simulation T(4) concentration was > or = 2.5 microg/dL and at least 1.5 times basal T(4) concentration.     

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.     

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.     

Thyrotropin-releasing hormone stimulation test to assess thyroid function in severely sick cats

Basal serum thyroxine (T4) concentration and the thyrotropin-releasing hormone (TRH) stimulation test were used to assess thyroid function in 36 critically ill cats examined between July 1996 and October 1998. Of the 36 cats. hyperthyroidism (as underlying or complicating disease) was suspected in 22 based on clinical signs, palpable thyroid nodules, and abnormal thyroid gland histology (study group). Hyperthyroidism was not suspected in the remaining 14 cats, which served as the control group. Based on serum T4 concentrations, suppression of thyroid function was documented in 14 (64%) cats of the study group and in 10 (71%) cats of the control group. The TRH stimulation test revealed an increase in serum T4 of less than 50% of the baseline concentration in 18 (82%) cats of the study group, and in 6 (43%) cats of the control group. In conclusion, based on the results of serum T4 determinations and the TRH stimulation tests, it was not possible to differentiate between cats with clinical and histologic evidence of thyroid dysfunction (hyperthyroidism) and cats with severe nonthyroidal illnesses.     

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.     

FELINE HYPERTHYROIDISM: EFFICACY OF TREATMENT USING VOLUMETRIC ANALYSIS FOR RADIOIODINE DOSE CALCULATION

Hyperthyroidism was diagnosed in 80 cats with thyroid scintigraphy using technetium pertechnetate. These cats were subsequently treated with radioiodine using a modified fixed dose method based on the volume of hyperfunctioning thyroid tissue calculated from the pertechnetate scans. The medical records and thyroid scintigrams were evaluated retrospectively. Follow-up was obtained on the cats to evaluate treatment success. Several parameters were evaluated in an attempt to identify a difference between treatment success and failure. Cats that failed to become euthyroid after one dose of radioiodine had a significantly higher pretreatment serum thyroxine level, had a significantly larger volume of hyperfunctioning thyroid tissue on scintigrams, and cats receiving oral versus intravenous radioiodine were over represented. Based on our results we conclude: 1) the administration of a dose of radioiodine based solely on the volume of hyperfunctioning thyroid tissue as estimated from the pertechnetate scan may be inadequate for those patients with extremely elevated serum thyroxine levels or large thyroid glands, and 2) oral administration of radioiodine is not recommended for the treatment of feline hyperthyroidism.     

Effect of storage of reconstituted recombinant human thyroid-stimulating hormone (rhTSH) on thyroid-stimulating hormone (TSH) response testing in euthyroid dogs

Bovine thyrotropin (bTSH) stimulation testing has long been considered the gold standard for diagnosis of canine hypothyroidism. Unfortunately, bTSH is no longer commercially available. Recently, the use of recombinant human thyrotropin (rhTSH) to perform thyroid-stimulating hormone (TSH) stimulation testing in dogs was described. The cost of an rhTSH vial (1.1 mg) limits the practical use of this product. The study reported here was performed to determine the effects of storing rhTSH on the post-TSH increase of serum total (TT4) and free (FT4) thyroxine concentrations during TSH stimulation testing in 12 euthyroid Beagles in a crossover trial. Three TSH tests with recombinant human thyrotropin (rhTSH; 91.5 microg IV) were performed on each dog during 3 different periods: 1 with freshly reconstituted rhTSH (fresh); 1 with rhTSH, reconstituted and stored at 4 degrees C for 4 weeks (refrigerated); and 1 with rhTSH, reconstituted and frozen at -20 degrees C for 8 weeks (frozen). Blood samples for determination of TT4 and FT4 concentrations were collected before and 4 and 6 hours after rhTSH administration. There was no significant difference in TT4 or FT4 concentration after stimulation with fresh, refrigerated, and frozen rhTSH. Furthermore, there was no significant difference between TT4 or FT4 serum concentration observed 4 and 6 hours after rhTSH administration. In conclusion, reconstituted rhTSH can be stored at 4 degrees C for 4 weeks and at -20 degrees C for 8 weeks without loss of biological activity, allowing clinicians to perform more TSH response tests per vial.     

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.