Several in-vivo studies have shown a procoagulant state in both overt and subclinical hyperthyroidism and in subclinical hypothyroidism. Insofar, no clinical studies have ever evaluated the relationship between thyroid dysfunction and clinically deep venous thrombosis (DVT). A pilot cross-sectional study aimed at assessing the frequency of overt and subclinical thyroid dysfunction patients with DVT was carried out. Fifty consecutive adult outpatients with a previous diagnosis of provoked DVT (pDVT), and 50 consecutive adult outpatients with a previous diagnosis unprovoked DVT (unDVT), both of the lower legs, who were followed at the Thrombosis Unit of the University Hospital of Varese, Italy, were enrolled after written informed consent. Fifty subjects, in whom such a diagnosis could be ruled out, served as controls. In each patient serum free thyroxine (FT4), free triiodothyronine (FT3), thyrotropin (TSH), anti-thyroid peroxidase (AbTPO), and anti-thyroglobulin (AbTg) antibodies were assayed. Previously unrecognised subclinical hypothyroidism was diagnosed in seven (14.0%) unDVT patients, one (2%) pDVT patient, and one (2%) control (odds ratio at multivariate analysis, 5.54; 95% confidence interval, 0.6-52.6); one new case of subclinical hyperthyroidism was diagnosed in each group; only one case (in the control group) of clinical overt hyperthyroidism was observed. The prevalence of thyroid autoantibodies, including both euthyroid and subclinical hypothyroid patients, did not differ in the three groups. The results of this pilot study suggest an increased prevalence of subclinical hypothyroidism in patients with unDVT. The clinical relevance of these preliminary findings needs to be addressed in larger prospective studies.