Hyperthyroidism is associated with a hypercoagulable state. Conversely, hypothyroidism is associated with reduced levels of von Willebrand factor and increased risk for bleeding. To investigate the relation between hyperthyroidism and risk for venous thrombosis, researchers in the Netherlands measured thyroid-hormone and thyroid-antibody levels in 190 patients with a first deep venous thrombosis (DVT) and 379 sex-matched controls.

The mean level of free thyroxine was significantly higher among DVT patients than among controls (16.2 pmol/L vs. 15.4 pmol/L; P<0.01), as was the mean T₃ level (1.90 nmol/L vs. 1.79 nmol/L; P<0.01). The higher the thyroxine level, the greater the risk for DVT, such that a level above the reference threshold (>24 pmol/L) was associated with an age- and sex-adjusted odds ratio for DVT of 13.0 (95% confidence interval, 1.1–154.1).

Thyrotropin levels tended to be higher in DVT patients than in controls, but they were not significantly associated with risk for DVT, particularly after adjustment for free thyroxine levels. Levels of thyroid peroxidase antibodies did not differ significantly between DVT patients and controls. Three DVT patients and no controls had overt hyperthyroidism (P=0.02).

Comment: The observational finding that increased thyroid activity is a risk factor for DVT is not intuitively obvious. Patients with hyperthyroidism usually have a hyperdynamic circulation rather than the sluggish blood flow that characterizes venous thrombosis. The investigators suggest that thyroid hormone–induced increases in factor VIII are responsible for the thrombophilia, but they did not use clotting-factor assays to confirm this. It is also possible that thyroid hormone is a surrogate for some other thrombogenic factor. Only large epidemiologic studies can confirm whether a high level of thyroid hormones is a risk factor for DVT. Until then, clinicians confronted with unexplained venous thrombosis might simply consider hyperthyroidism as a possibility.

— David Green, MD, PhD

Published in Journal Watch Oncology and Hematology June 29, 2010