What Causes the Increase in Platelet Mean Volume in Thyroid Pathological Conditions?

Hyperthyroidism and venous thrombosis: a casual or causal association? A systematic literature review

A kaleidoscope of coagulation disorders have been reported in patients with thyroid dysfunctions. Globally, these disorders involve both primary and secondary hemostasis and range from subclinical laboratory abnormalities to, more rarely, life-threatening hemorrhages or thrombotic events. While overt hypothyroidism appears to be associated with a bleeding tendency, hyperthyroidism emerged to have an increased risk of thrombotic events. In particular, a number of case reports have documented acute venous thrombosis complications in patients with overt hyperthyroidism, especially at cerebral sites. Nevertheless, further observational and intervention studies might be needed to provide a more definitive information on the clinical relevance of this association, along with the potential implication for prevention and treatment of coagulation-fibrinolytic abnormalities in patients with thyroid dysfunction.

The effect of subclinical hypothyroidism on platelet parameters

BACKGROUND

Hypothyroidism has a broad clinical spectrum. Today, physicians frequently encounter patients with very mild thyroid dysfunction instead of overt hypothyroidism. These patients have normal serum levels of thyroxine and triiodothyronine and only mildly elevated serum thyrotropin levels. Such patients are often identified through routine screening or in the course of an evaluation of common nonspecific symptoms. On the other hand, coronary heart disease is the leading cause of death in developed countries. There are studies, which suggest platelets play a role in the pathogenesis of atherosclerosis and coronary heart disease.

AIM

The aim of this study is to compare the platelet count and other platelet parameters in subclinical hypothyroidic and euthyroidic healthy control group and to investigate whether these parameters have a predictive significance in patients with subclinical hypothyroidism.

MATERIALS AND METHODS

Forty-seven patients with subclinical hypothyroidism and 30 euthyroidic healthy control group were enrolled into the study.

RESULTS

Patients with subclinical hypothyroidism had higher mean platelet volume (MPV) and platelet distribution width (PDW) values than control group, which were statistically significant (p<0.001 and p<0.001), respectively.

CONCLUSION

Our results indicate that MPV and PDW play an important predictive role in subclinical hypothyroidism.

Thyroid, hemostasis and thrombosis

The aim of this paper is to briefly review some practical aspects of the relationship between thyroid function and several disorders of the hemostatic system in terms of bleeding and thrombosis. Thrombocytopenia, acquired hemophilia, hypercoagulability, cardioembolism and other biochemical coagulative and fibrinolytic abnormalities have been described in the past years both in hyper- and hypothyroidism. Since most of hyper- and hypothyroid conditions are the consequence of autoimmune thyroid disease (1), either deranged immune function, altered circulating thyroid hormone concentration, or both may concur in the pathogenesis of hemostatic disorders of potential crucial clinical impact. These aspects will be outlined and discussed in an attempt to give answers to some questions, often arising in the clinical approach.

Ultrastructural morphometric analysis of human blood platelets exposed to minimal handling procedures

A detailed morphometric study of normal human blood platelets is described. The purpose has been to evaluate the morphological characteristics of platelets exposed to minimal handling procedures in order to obtain an optimal basis for the appraisal of platelets in disease. Blood from 10 healthy volunteers was collected directly into buffered glutaraldehyde and processed for electron microscopy. This platelet fixation procedure resulted in excellent preservation of resting platelet ultrastructure with one exception: the dense bodies. Compared to platelets fixed following washing procedures, our directly fixed platelets comprised fewer pseudopods and contained more glycogen. An unexpected feature of the open canalicular system was the apparent release of blisters interpreted as microvesicles. Employing a computerized image analyzer, 300 of the platelets were examined morphologically. The morphometric data thus obtained were analyzed statistically, resulting in a set of standard values for morphological characteristics of human platelets which we have found useful in subsequent evaluations of platelet morphology in disease. Significant inter-individual variance was, however, detected in two instances, in the section area of the alpha granules, as well as the area fraction of platelet sections occupied by channels of the open canalicular system (OCS). This should be taken into consideration when appraising platelet ultrastructure in health and disease.

Model for the regulation of platelet volume and responsiveness by the trans-membrane Na+/K(+)-pump

The correspondence between K+ uptake in platelets to their responsiveness was studied using 86Rb+ as an analogue of K+. An average 86Rb+ uptake rate of 0.73 (+/- 0.140) x 10(-15) mole Rb+/min-plt (n = 20) was observed. By the use of K(+)-influx inhibitors, we were able to distinguish three distinct 86Rb+ uptake pathways: an ouabain-sensitive (61% +/- 2% inhibitable) pump and two equivalent channels, only one of which is sensitive to furosemide. Other platelet parameters were also examined in conjunction with K(+)-uptake. Platelets incubated with ouabain exhibited an overall rise in their cell volume (MPV) with incubation time (delta MPV = 7.4 x 10(-17) L/min-1 plt-1). Concomitantly, over 24 hours, a steady decrease in platelet number was recorded by blood cell coulter, which correlated inversely with the counts of particles, which by their size resemble white blood cells (r = 0.89). On a cellular level, incubation with ouabain induced greater expression of surface fibrinogen-receptor (GPIIb), increased binding of FITC-labelled fibrinogen, and increased responsiveness to ADP. Our observations suggest the following sequence of events: Ouabain turns off the Na+/K(+)-ATPase pump, which leads to water accumulation in platelets and concomitant increased MPV. Greater expression of fibrinogen receptors on the distended platelet surface corresponds to spontaneous microaggregate formation as well as greater responsiveness to agonists. Our model links volume regulation, the expression of fibrinogen receptors, and the sensitivity of platelets to agonists to the activity of the Na+/K(+)-ATPase pump.