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Platelet function in cats with hyperthyroidism

Hiebert EC, Panciera DL, Boes KM, Bartl L. Platelet function in cats with hyperthyroidism. J Feline Med Surg [Internet]. 2020 May 21.

Hyperthyroidism is commonly encountered in geriatric cats and is associated with a myriad of clinical signs including weight loss, cardiac, renal, and hepatic dysfunction, and thromboembolic events. Aortic thromboembolism is encountered in a subset of cats with heart disease, but may be seen in hyperthyroid cats in the absence of structural cardiac changes. This suggests that cats with hyperthyroidism may exist in a hypercoagulable state. In humans with hyperthyroidism, increases in von Willebrand factor, factor VIII, factor IX, and others, combined with increased platelet aggregation, contribute to an increased risk of ischemic stroke. The purpose of this study was to determine if platelet function is altered in cats with hyperthyroidism compared to euthyroid cats.

A prospective, case control observational study design was used including euthyroid cats presenting to a veterinary teaching hospital for preventative care and compared to hyperthyroid cats. Control cats were >8 years old and had normal CBC, biochemistries, TT4, and physical exam findings. Hyperthyroid cats were required to have no concurrent disease and received no medications for two weeks prior to diagnosis. Cats with microscopic platelet clumping were excluded from analysis. Platelet function was assessed using the PFA-100 platelet function analyzer using a combined collagen and ADP cartridge.

Ten control and twenty hyperthyroid cats were recruited. One control cat was excluded due to lack of blood film evaluation. Four hyperthyroid cats were excluded due to platelet clumping. The control group consisted of nine cats, four of which were neutered males and five spayed females. The hyperthyroid group comprised sixteen cats, seven neutered male and nine spayed female. Mean age and body weight were the same between groups.

Three of the hyperthyroid cats had received no treatment prior to the study, thirteen received methimazole, and two an iodine restricted diet. All of these treatments were discontinued at least two weeks prior to the study. The mean TT4 in the hyperthyroid group was 146nmol/L, compared to 25nmol/L in the control group.

The estimated platelet count on blood film was not different between groups, however automated CBC on EDTA blood was significantly higher for the hyperthyroid group. Mean platelet volume did not differ. Closure times on the PFA-100 (a measure of platelet function, with lower closure times associated with increased platelet function) did not differ between hyperthyroid or control groups.

The authors of this study conclude that platelet function is not affected by hyperthyroidism in cats. While this differs from the results of a previous, smaller study that indicated an alteration in platelet function, the authors point out several limitations to the previous study (ie small sample size, concurrent cardiac disease, beta blocker use).

There were a few limitations to this study. Sample sizes were small, and potentially underpowered to detect small differences in platelet function. While the PFA-100 is a test of platelet function, it is more reliable for detecting decreased compared to increased function. Further, the PFA-100 does not assess the entire coagulation system, and so measurement of standard coagulation tests (ie PT/PTT), coagulation factors (i.e. vWF), and assessments of global coagulation (i.e. TEG, ROTEM), may have provided further insight into a potential hypercoaguable state in cats with hyperthyroidism.

Overall, this study serves to show that hyperthyroidism by itself does not alter platelet function in cats as measured by a collagen-ADP cartridge on the PFA-100. Whether a hypercoagulable state exists in these cats remains to be determined. (MRK)

See also:

Borgeat K, Wright J, Garrod O, et al. Arterial thromboembolism in 250 cats in general practice: 2004–2012. J Vet Intern Med. 2014; 28: 102–108.

Ho KK, Abrams-Ogg AC, Wood RD, et al. Assessment of platelet function in healthy sedated cats using three whole blood platelet function tests. J Vet Diag Invest. 2015; 27: 352–360.