Hyperthyroidism as a Risk Factor for Venous Thromboembolism: A Case-Control Study

<>BACKGROUND: It has been hypothesized that overt hyperthyroidism may increase venous thromboembolic risk given substantiated evidence of a thyroid-induced hemostatic imbalance favoring a hypercoagulable state. Aim of this study was to investigate whether or not overt hyperthyroidism functions as a risk factor for venous thrombosis.

<>METHODS: We performed laboratory measurement of thyroid function in 190 patients with objectively confirmed venous thrombosis of the lower extremities and 375 sex-matched controls drawn from a prospective study on general risk factors for venous thrombosis.

<>FINDINGS: Of the 190 included cases, 155 patients were diagnosed with deep venous thrombosis (DVT), 12 patients with calfvein thrombosis and 23 with superficial thrombophlebitis. Undetected overt primary hyperthyroidism was found present in 3 DVT patients at the time of the event, whereas none was seen in controls (p=0.037, all thrombosis patients combined; p=0.024, DVT patients only). We found slightly higher levels of serum free thyroxine (fT4) in patients with venous thrombosis as compared to controls, and analysis of the proportion of cases with a higher fT4, above 16 pmol/L (75^th percentile of values in controls), yielded an odds ratio of 1.7 for DVT to develop (95% CI 1.12–2.45 when adjusted for sex, age and known thyroid dysfunction). No such association was found for serum levels of thyrotropin or thyroidperoxidase antibodies.

<>INTERPRETATION: Our data suggest overt hyperthyroidism to function as a risk factor for venous thrombosis. In addition, higher levels of fT4, even within the normal range, are associated with increased VTE risk.