Serum Thyroid Hormone Concentrations Fluctuate in Cats with Hyperthyroidism

Transcription

1 Serum Thyroid Hormone Concentrations Fluctuate in Cats with Hyperthyroidism Mark E. Peterson, DVM, Thomas K. Graves, and lrena Cavanagh We measured serum thyroxine (T4) and 3,3,5triiodothyronine (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 10hour study period the coefficient of variation (CV) for serum T4 and T3 concentrations ranged from % (mean = 12.0 k 4.8%) and from % (mean = 17.5 k 6.3%), respectively. During the 15day study period, CV for serum T4 ranged from % (mean = 18.4 k 9.3%), while CV for serum T3 ranged from % (mean = 20.1 k 8.6%). These CV values were significantly higher than the expected intraassay 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 10hour 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. (Journal of Veterinary Internal Medicine 1987; 1 : ) HYPERTHYROIDISM is the most commonly diagnosed endocrinopathy. in feline practice, and, as such, has become an area of intense interest for small animal veterinarians and research scientists. Clinical signs and results of physical examination (eg, palpable thyroid nodule) are useful in the tentative diagnosis of feline hyperthyroidism. However, since many of the clinical signs associated with hyperthyroidism also are found in a variety of other disorders of older cats, the definitive diagnosis usually relies on demonstration of abnormally high serum concentrations of thyroid hormone.14 Other diagnostic modalities, such as radionuclide thyroid imaging or radioiodine thyroidal uptake determination~,**~ can also be of use in confirming hyperthyroidism in cats, but are not widely available for most small animal practitioners. While the finding of high serum concentrations of thyroxine (T4), 3,3,5triiodothyronine (T3), or both, is From the Departments of Medicine (Peterson, Graves) and Pathology (Cavanagh), The Animal Medical Center, 5 10 East 62nd Street, and the Research Animal Resource Center (Peterson, Graves), Cornell University Medical College, 1300 York Avenue, New York, NY Reprint requests: Dr. Mack E. Peterson, The Animal Medical Center, 510 East 62nd Street, New York, NY the biochemical hallmark of hyperthyroidism and can confirm a tentative clinical diagnosis, it has not been determined if the finding of normal thyroid hormone concentrations in cats in which hyperthyroidism is suspected (based on clinical signs and physical examination findings) can be used to exclude a diagnosis. Variation in serum or plasma concentrations of the thyroid hormones over time has been documented in normal maq5v6 rat, and dog,* as well as in human patients with hyperthyroidism caused by thyroid aden~ma.~ If circulating thyroid hormone concentrations in hyperthyroid cats are subject to a significant degree of fluctuation, it is conceivable that cats with mild disease could show normal serum T4 or T3 (or both) values if sampled at a given time. The purpose of this study, therefore, was to determine the degree of serum T4 and T3 fluctuation in cats with mild to moderate hyperthyroidism, thereby determining whether the finding of normal values could obfuscate the diagnosis of hyperthyroidism in cats with consistent clinical signs and physical eramination findings. Materials and Methods Fourteen cats with untreated hyperthyroidism examined at The Animal Medical Center were used in this 142

2 Vol. 1. NO. 3 SERUM THYROID HORMONE CONCENTRATIONS 143 study. These cats ranged in age from 918 years (mean t SD = 13.8 f 2.7 years). Thirteen cats were of mixed breeding (domestic short and longhaired) and the remaining cat was Siamese; seven of the cats were male and seven were female. Most of the cats were selected for study because of the presence of a relatively mild hyperthyroid state. All of the cats, however, had clinical signs of hyperthyroidism (eg, weight loss, polyphagia, vomiting, diarrhea) as well as the finding of palpable thyroid nodule(s) on physical examination. The diagnosis of hyperthyroidism was confirmed in all cats by demonstrating elevated serum concentrations of thyroxine (T4), triiodothyronine (T3), or both, as previously described. All 14 cats had 10 blood samples collected at hourly intervals (i.e., from 9:OO 18:OO hrs) throughout one day for determination of serum T4 and T3 concentrations. Seven of these cats (nos. 17; Tables 1, 2) were studied further by collecting additional morning samples (approximately 2 ml of blood per sample) for serum T4 and T3 determinations once daily during the next consecutive 14day period. For the 10hour study, blood samples were collected through an indwelling, 18gauge catheter, which was placed in a saphenous vein of each cat prior to the testing period. Following completion of this 10hour testing period, the catheters were removed and, in the seven cats studied over the 15day period, the remaining blood samples were collected by jugular venipuncture. The blood samples were centrifuged within two hours of collection; the serum was separated and frozen until assayed. Serum T4 and T3 were measured using commercial radioimmunoassay (RIA) kits (Micromedic T4 and T3 RIA Kits, Micromedic Systems, Horsham, Pa.) The antit4 antibody had a 0.4% crossreactivity with T3 and a 0.4% crossreactivity with 3,3,5 T3 (reverse T3). The antit3 antibody crossreacted 0.13% with T4, 0.19% with 3,5,diiodoLthyronine, and 0.01% with reverse T3. Assays of serial dilutions of a feline serum pool containing high concentrations of the thyroid hormones resulted in inhibition curves with slopes parallel to the standard curves for T4 and T3. Accuracy of the assays was determined by adding varying quantities of T4 and T3 to a feline serum pool containing undetectable concentrations of the hormones. Linear regression analysis of the resulting data (x = amount of hormone added; y = amount of hormone measured) gave the equations 0.96~ (correlation coefficient, 0.99) for the T4 assay and 0.97~ (correlation coefficient, 0.99) for the T3 assay. The sensitivity of the T4 and T3 assays was 0.2 pg/dl and 15 ng/dl, respectively. The intraassay coefficient of variation for the T4 assay was 5.1%, calculated by assay of 10 replicates of three feline serum pools containing hormone concentrations of 2.3 f 0.1 pg/dl, 5.1 k 0.2 pg/dl, and 15.2 f 1.1 pg/dl. For the T3 assay, intraassay coefficient of variation for 10 replicates of TABLE I. Analysis of Serum T4 and T3 Values Determined Hourly over 10 Hours in 14 Cats with Hyperthyroidism Serum T4 (pg/dl) Serum T3 (ngldl) Cat No. Mean f SD Range CV Mean f SD Range CV k f * f k * f f f f f i CV = coefficient of variation f f f f f f f f f f f three feline serum pools containing hormone concentrations of 61 f 5.9 ng/dl, 113 f 9.5 ng/dl, and 238 f 13.7 ng/dl was 7.7%. The interassay coefficients of variation for the T4 and T3 assays (using three serum pools with medium to high hormone concentrations run in 20 consecutive RIAs) were 10.2% and 10.4%, respectively. All results are given as the mean f SD. Variations in serum T4 and T3 concentrations in the cats were analyzed by calculating the coefficient of variation (CV) in hormone values for both the 10hour and 15day study periods. As with the assay validation procedure, the CV for the serum T4 and T3 concentrations for each cat was calculated using the following formula: CV(%) 100 X SD. The mean values for T4 and T3 from the mean 10hour testing period (in cats nos. 17) were used as the values for day 1 of the 15day testing period. For each cat, all serum samples from both the 10hour and 15 day testigg periods were determined (in duplicate) in the same assay to exclude the effect of interassay variation in hormone concentrations. Statistical analyses were TABLE 2. Analysis of Serum T4 and T3 Values Determined Daily over 15 Days in Seven Cats with Hyperthyroidism Serum T4 (pg/dl) Serum T3 (ng/dl) Cat No. Mean k SD Range CV Mean f SD Range CV f f f ? f f f f fo f : CV = coefficient of variation.

3 144 PETERSON ET AL Journal of Veterinary Internal Medicine 8 7? b \ (51 a 4 w t 3 E 2 2 m 0' " " " 1 I I I lo FIG. 1. Serum T4 and T3 concentrations determined hourly over a 10hour period in a cat with hyperthyroidism (cat no. 3, Table 1). Note the fall of serum T3 values into the normal range during the 8th10th hour. performed by using Student's paired or unpaired ttest and analysis of variance, as appropriate. lo Correlations between circulating T4 and T3 concentrations were calculated by linear regression analy~is.~ For all statistical analyses, a P value of 0.05 or less was considered significant. Results During the 10hour sampling period, the individual CV for serum T4 from all 14 of the hyperthyroid cats (range = 6.4 to 22.6%; mean = 12.0 f 4.8%) was higher than the intraassay CV of 5.1% (Table 1). Similarly, the CV for serum T3 from these cats (range = 9.6 to 33.1%; mean = 17.5 k 6.3%) was higher than the intraassay CV of 7.7% (Table 1). Three of the cats (nos. 2,8,9; Table 1) with mild clinical and biochemical features of hyperthyroidism had mean serum T4 values within the highnorma1 range (normal = pgldl), whereas six cats (nos. 2,5,6,9, 11, 14; Table 1) had normal mean serum T3 concentrations (normal = ng/dl). Of the 10 serum thyroid hormone determinations, 5 of the 14 cats had 18 T4 values that were within normal range. Nine of the cats had 110 normal serum T3 concentrations during this study period. Serum T4 and T3 fluctuations in three cats are shown in Figures 13. When serum thyroid hormone concentrations from all 14 cats were averaged for each individual sampling time, there was no significant fluctuation in values over the 10hour period, with mean serum concentrations for T4 and T3 ranging from 6.8 f 4.3 pg/dl to 7.7 f 5.9 pg/dl and ng/dl to ng/dl, respectively. The times of the highest and lowest serum T4 and T3 concentrations were extremely variable among individual cats. During the 15day sampling period, the CV for serum T4 from all seven of the hyperthyroid cats (range = 6.6 to 34.8%; mean = 18.4 f 9.3%) was higher than the intraassay CV for T4 (Table 2). Similarly, the CV for serum T3 from these cats (range = %; mean = 20.1 f 8.6%) was higher than the intraassay CV for T3 (Table 2). Two of the cats (nos. 2, 6; Table 2) with mild clinical and biochemical features of hyperthyroidism had mean serum T4 values within the highnormal range, whereas four cats (nos. 2, 3, 5, 6; Table 2) had normal mean serum T3 concentrations. Of the 15 serum thyroid hormone determinations, three of the seven cats studied had 61 1 serum T4 values that were within normal range. Four of the seven cats had between 1 and 15 normal serum T3 concentrations during the 15day period. Figures 46 show serum T4 and T3 fluctuations in cats No. 3, 4 and 7. In two of the seven cats (nos. 3, 7; Tables 1, 2), the variations in serum thyroid hormone concentrations were greater during the 15day period than during the 10hour daily sampling period (Figs. 1, 37 4, 6). During the 10hour sampling period, there was a significant correlation (R = 0.69 to 0.93) between individual cat serum T4 and T3 concentrations in 1 1 of the 14 cats. Likewise, a significant correlation (R = 0.61 to 0.97) between individual serum T4 and T3 values existed in six of the seven cats sampled over the 15day period. Overall, the correlation between T4 and T3 concentrations in all 14 cats from all sample times was highly significant (R = 0.90; P < 0.001) FIG. 2. Serum T4 and T3 concentrations determined hourly over a 10hour period in a cat with hyperthyroidism (cat no. 4, Table 1). FIG. 3. Serum T4 and T3 concentrations determined hourly over a 10hour period in a cat with hyperthyroidism (cat no. 7, Table 1).

4 Val. 1 ' NO. 3 SERUM THYROID HORMONE CONCENTRATIONS 145 Discussion The results of this study indicate that thyroid hormone concentrations in hyperthyroid cats are subject to a degree of fluctuation that exceeds normal assay variation. In cats with serum T4 and T3 values well above that normal range, this thyroid hormone fluctuation is probably not of great clinical or diagnostic significance. However, in cats with mild hyperthyroidism, the degree of serum T4 and T3 fluctuation that can occur, as well as our finding of a relatively high number of normal results in these cats, suggests that a diagnosis of hyperthyroidism can not be excluded on the basis of the finding of a single normal to highnormal serum T4 (or T3) result alone. In cats with clinical signs consistent with hyperthyroidism (and especially in cats with palpable goiters), it is possible that more than one serum T4 (and T3) determination could be required to confirm a diagnosis. In addition, our finding of more pronounced thyroid hormone fluctuations over the 14day study period vs. the 10hour period in some hyperthyroid cats suggests that multiple blood samples for these thyroid hormone determinations should be collected on separate days rather than at different times during the same day. In this study, there did not appear to be any consistent time during the day when thyroid hormone concentrations were at their highest, indicating that there is no specific time at which blood sampling for serum thyroid hormone determinations are recommended. Although the presence of a circadian rhythm in thyroid hormone secretion can neither be confirmed nor excluded on the basis of this study, the lack of any notable overall trend in serum T4 and T3 values during our 10hour sampling period is consistent with other findings in normal human^,^,^ rats,7 and dogs.* There are a number of possible mechanisms for the serum T4 and T3 fluctuations in the cats of this study. Since the biological halflives of circulating T4 and T3 FIG. 5. Serum T4 and T3 concentrations determined daily over a 15day period in a cat with hyperthyroidism (cat no. 4, Table 2). appear to be relatively long (hours) in the hyperthyroid cat," it is unlikely that these fluctuations result entirely from changes in thyroid secretion. Changes in thyroid hormone secretion could contribute to increases in serum thyroid hormone values, but decreases in T4 or T3 secretion alone could not explain the acute decreases in circulating values. Similarly, it is unlikely that serum thyroid hormone fluctuations could be the result of shortterm pulses in pituitary thyroid stimulating hormone (TSH) secretion, since it is likely that the excessive circulating levels of thyroid hormone suppress serum TSH values in cats with hyperthyroidism, as occurs in human patients with thyrotoxic~sis.'~*~~ It has also been suggested that the shortterm changes in serum thyroid hormone concentrations could result from fluctuations in plasma proteins (a large percentage of circulating T4 and T3 are proteinbound), or from hemodynamic Overall, the underlying mechanism of serum thyroid hormone fluctuation, both in normal and hyperthyroid states, remains unknown. In most of the cats of this study, there was a good correlation between serum T4 and T3 concentrations in both the 10hour and 15day studies. However, normal 1 b 6t t 'b R i 1 R 1250 O' ; h i k A ; b b IIO III 1; 1; Ib 1; '0 Time (days) FIG. 4. Serum T4 and T3 concentrations determined daily over a 15day period in a cat with hyperthyroidism (cat no. 3, Table 2). Note the fluctuation in and out of the normal range for both serum T4 and T3 values. FIG. 6. Serum T4 and T3 concentrations determined daily over a 15day period in a cat with hyperthyroidism (cat no. 7, Table 2). Note the serum T3 fluctuation into the normal range on days 7 and 13.

5 I ~~ ~ 146 PETERSON ET AL Journal of Veterinary Internal Medicine serum T3 values were observed more frequently in these cats than were normal circulating T4 concentrations. This finding supports our previously reported observation that a serum T4 determination is probably of greater diagnostic value than determination of serum T3 Although the reason for normal serum T3 concentrations in some mild to moderate cases of feline hyperthyroidism is unclear and there are many possible explanations, it is likely that these values would have eventually risen to within the hyperthyroid range if the disorder was left untreated. References 1. Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J Am Vet Med Assoc 1983; 183: Peterson ME. Feline hyperthyroidism. Vet Clin North Am: Small Anim Prac 1984; Peterson ME, Turrel JM. Feline hyperthyroidism. In: Kirk RW, ed. Current Veterinary Therapy IX. Philadelphia: WB Saunders, 1986; Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Vet Radio1 1984; 25: OConnor JF, Wu GY, Gallagher TF, Hellman L. The 24hour plasma thyroxin profile in normal man. J Clin Endocrinol Metab 1974; 39: Azukizawa M, Pekary AE, Hershman JM, Parker DC. Plasma thyrotropin, thyroxine and triiodothyronine relationships in man. J Clin Endocrinol Metab 1976; Fukuda H, Greer MA, Roberts L, et al. Nyctohemeral and sexrelated variations in plasma thyrotropin, thyroxine and triiodothyronine. Endocrinol 1975; 97: Kemppainen RJ, Sartin JL. Evidence for episodic but not circadian activity in plasma concentrations of adrenocorticotrophin, cortisol and thyroxine in dogs. J Endocrinol 1984; 103: Gemsenjager E, Staub JJ, Girard J, Heitz PH. Preclinical hyperthyroidism in multinodular goiter. J Clin Endocrinol Metab 1976; 43: Glantz SA. Primer of Biostatistics. New York: McGrawHill, Peterson ME, Becker DV. Spontaneous hyperthyroidism in the cat: An animal model for toxic nodular goiter. Proceedings of the 59th Annual Meeting of the American Thyroid Association, 1983; T Ingbar SH. The thyroid gland. In: Wilson JD, Foster DW, eds. Textbook of Endocrinology, 7th ed. Philadelphia: WB Saunders, 1985; Larsen PR. Thyroid hormone concentrations. In: Ingbar SH, Braverman LE, eds. The Thyroid, 5th ed. Philadelphia: JB Lipincott, 1986; CLASSIFIED AD SECTION JVlM makes available classified advertising space. Each line is $8.00. The average line will be 55 characters. Service charge for a blind box is $6.50. Advertising copy is subject to acceptance by the publisher. JVlM is a quarterly journal published in February, May, August and November. Copy must be received by the 1st of the month preceding the month of the issue. Send advertisements, TYPED to: JOURNAL OF VETERINARY INTERNAL MEDICINE J.B. Lippincott Company Attention: Joan McFetridge East Washington Square Philadelphia, Pa (215) Display space such as l/4 and /2 page is also available for those desiring greater visibility FOCUSING ENDOSCOPESThe New Focuscope Modular Endoscopelotoscope System offers many features found only in today s most expensive instruments! Write for informationmedical DIAGNOSTIC SERVICES, Inc. P.O. Box 17326/Airport Station, th Terrace North, Clearwater, FL 33520, Phone: European Society of Veterinary Neurology (ESVN) Inaugural Meeting November 27th 1987, Paris, France. Held in conjunction with the Conference Nationale des Veterinaires Specialises en Petits Anirnaux (CNVSPA) at La Maison de la Chimie. Contact Dr. Philippe Moreau, Organizing Committee, 69 rue VictorienSardou, Limoges, France, Tel. (33) or Dr. Gerl W. Niebauer, University of Pennsylvania, 3850 Spruce Street, Philadelphia, PA , Tel. 215/