In addition, a significantly higher percentage of cats with a total T4 concentration below the reference range had a TSH concentration above the reference range (68 26 of 38 cats) than did cats with a total T4 concentration within the reference range (17 15 of 87 cats).Nonthyroidal illness (NTI) and age are not expected to increase TSH concentrations in the majority of patients. In dogs, for example, approximately only 10 of euthyroid patients with nonthyroidal disease have increased TSH concentrations 14 ; euthyroid cats with chronic kidney disease do not have increased TSH concentrations.
10 If high TSH concentrations are common in cats receiving methimazole treatment and the combination of a low total T4 or fT4ed concentration together with a high TSH concentration is associated with azotemia, veterinarians should routinely measure TSH concentrations in cats receiving methimazole, which is not the current practice.The primary objectives of this study were: (1) to determine the prevalence of IH in an independent study with a large number of cats receiving methimazole treatment, (2) to compare the ability of total T4 and fT4ed concentrations in conjunction with TSH concentrations to define thyroid function status in hyperthyroid cats receiving methimazole, and (3).
Of those cats with a low total T4 or fT4ed concentration, approximately 70 had an increased TSH concentration. Overall, measurement of fT4ed concentration was not a better indicator of the presence of IH than was total T4 concentration in cats being treated with methimazole.To our knowledge, only a single study has measured total T4 and TSH concentrations in cats receiving medical treatment for hyperthyroidism. Surprisingly, 48 of cats had a combination of low total T4 and increased TSH concentrations, 5 suggesting IH may be common.
Whether or not correction of IH also will improve survival remains to be determined. In a previous study 5 as well as ours, TSH concentrations were increased in a subset of patients that had total T4 and fT4 concentrations within the reference range.Of cats with an increased TSH, the percentage of cats with a low total T4 concentration was not different than the percentage of cats with a low fT4ed concentration. Cats with an increased TSH concentration were more likely to be azotemic than cats with a normal TSH concentration.
18 However, a heterologous assay has been validated in cats and the results correlate clinically with total T4 concentrations. 5, 10, 18 In this study, TSH concentrations in cats with a total T4 concentration below the reference range were significantly higher than those in cats with a total T4 concentration within the reference range.Accordingly, fT4ed concentration would be below the reference range in more cats with an increased TSH concentration as compared to total T4 concentrations, thus identifying more cats with IH than identified by total T4.
5 Correction of IH caused a significant decrease in serum creatinine concentration and, in 50 of cats, resolution of azotemia. 9 Thus, monitoring for IH by measurement of both total T4 and TSH concentrations is crucial to minimize azotemia.The prevalence of IH has not been confirmed in an independent study, to our knowledge. Measurement of serum total thyroxine concentration (T4) is commonly used for therapeutic monitoring. Total T4 concentrations can be difficult to evaluate, however.
Similarly, previous studies found TSH concentrations to fit expectations, with low TSH concentrations in hyperthyroid cats 10, 19 and high TSH concentrations in cats previously treated with radioactive iodine. 5 Thus, measurement of TSH concentrations in cats is clinically useful.In a study, cats with IH were more likely to be azotemic, and the median survival time of hypothyroid azotemic cats (456 days) was significantly shorter than that of hypothyroid nonazotemic cats (975 days).
Because hyperthyroidism can lead to atrophy of pituitary thyrotrophs, patients early in the course of methimazole treatment still may have suppressed TSH concentrations. 5 In this study, cats in the first 13 months of treatment may have had TSH concentrations within reference range despite low total T4 or fT4ed; with more time TSH concentration may have.The percentage of cats with azotemia did not change in relation to total T4 or fT4ed concentrations (ie, whether they were within the reference range or not). The development of a low total T4 concentration but not of azotemia was related to methimazole dose.
10-13 Serum free thyroxine concentration as measured by equilibrium dialysis (fT4ed) may be a better marker of thyroid function than total T4 concentration. 11. To our knowledge, fT4ed concentrations during treatment with methimazole have not been evaluated to determine if they are easier to interpret than total T4 concentrations.Description, VWR Catalog Number, Supplier Number, Unit, Price, Quantity. L-. THYROXINE SODIUM PENTAHYDRATE LG, TT0160-100, Each.
Iatrogenic hypothyroidism potentially could be defined best as the presence of a low serum thyroid hormone concentration in combination with the appropriate physiological response of an increased TSH concentration. Clinical hypothyroidism can be difficult to judge, because after resolution of hyperthyroidism, cats are expected to sleep more, eat less and gain weight, signs also consistent.We further hypothesized that fT4ed concentrations would more closely correlate (negatively) with serum creatinine concentration as compared to total T4 or TSH. Discussion Top of page Abstract Materials and Methods. Results Discussion Acknowledgments References In this study, we determined that approximately 20 of cats had IH if defined as a combination of a low thyroid.
Unfortunately, given the retrospective nature of our study, information on other medications being administered that could suppress thyroid hormone concentrations or the presence of NTI was not available. Given the detrimental effects of hypothyroidism on renal function and survival identified in other studies, consideration should be given to lowering the methimazole dose in cats with.The prevalence of IH reported previously 5 and that reported in this study likely are underestimated. Hypothyroidism potentially exists in some cats with low total T4 and fT4ed concentrations but a TSH concentration within the reference range.
The overall percentage of cats with an increased TSH concentrations in this study was 33 (41/125 which is similar to the 35 determined in previous studies of cats t).8 On the other hand, treatment of IH may improve renal function. 9 Despite the potential link between IH and azotemia, the best means of monitoring cats after treatment has not been determined.
Levothyroxine (INN, USAN ) or L-thyroxine is a synthetic thyroid hormone that is chemically identical to thyroxine (T4 which is naturally secreted by the follicular).Furthermore, our results suggest that, even given the limitations of the TSH assay in cats, a high percentage of cats receiving methimazole that have low total T4 concentrations may be hypothyroid as compared to having low total T4 concentrations for other reasons such as NTI; NTI most commonly is described with decreases in total T4.
The TSH assay, developed for dogs, even fails to recognize hypothyroidism in dogs. Unfortunately, no feline-specific TSH assay has been developed and the canine assay is insensitive, measuring approximately 35 of feline TSH.5 Approximately 1517 of cats receiving methimazole of cats treated with radioactive iodine (131I) or bilateral thyroidectomy became azotemic within 6 months of treatment initiation. 2-7 Pre-existing azotemia can decrease overall survival times in cats treated for hyperthyroidism.
Quantification of total thyroxine in plasma from Xenopus laevis. Luna LG(1 Coady K, McFadden JR, Markham DA, Bartels MJ).A secondary objective was to evaluate a possible association between the dose of methimazole and IH and azotemia. We hypothesized that serum fT4ed concentrations would be a better marker of thyroid function than total T4.