Hyperthyroidism enhances platelet plug formation

Objective: Several studies have reported abnormalities of blood coagulation parameters in hyperthyroidism including an increase of von Willebrand Factor (vWF). As vWF plays an important role in primary hemostasis, we hypothesize that hyperthyreosis induced VWF increase leads to platelet activation and therefore to enhanced platelet plug formation.

Methods: Thirdy patients with hyperthyroidism (T4 175±15 ng/ml, T3 4.1±0.6, TSH 0.01±0.02µU/ml) and 30 controls (TSH 1.44±0.34µU/ml) were included. Patients with hyperthyroidism were treated with 10–60mg/day thiamazole according to thyroid function. vWF and bleeding time (collagen epinephrine induced closure time; platelet function analyzer PFA-100) were measured at baseline and during therapy with thiamazole. All resullts are reported as mean±95% confidence intervals.

Results: In patients with hyperthyroidism baseline vWF was higher than in controls (278±36% vs. 124±16%; p<0.001). High vWF-Ag levels were associated with shorter baseline bleeding times in patients with hyperthyroidism compared to controls (114±9s vs. 130±10s; p=0.01). During therapy with tiamazole T4 decreased from 175±15 ng/ml to 90±20 ng/ml (after 8 weeks), vWF levels decreased to normal values (160±30%; p<0.01 vs. baseline), and bleeding time was prolonged compared to baseline (156±20s p<0.01).

Conclusion: Hyperthyroidism induced vWF increase is associated with platelet activation and therefore shorter bleeding times. Platelet plug formation, which has been shown to predict to predict cardiovascular risk, decreases during therapy with thiamazole. Hence, our study provides a further explanation for the increased cardiovascular risk by hyperthyroidism.