Hyperthyroidism is associated with shortened APTT and increased fibrinogen values in a general population of unselected outpatients.

Although clinical evidence supports a plausible association between thyroid disorders and venous thrombosis, reliable evidence is so far lacking on the potential biological mechanisms and neither temporary nor permanent thromboembolic risk factors were constantly identified. We performed a 2-years retrospective analysis to retrieve combined results of activated partial thromboplastin time (APTT), prothrombin time (PT), fibrinogen, and thyroid stimulating hormone (TSH), performed on outpatients over 15 years of age and referred by general practitioners for routine blood testing. Cumulative results were retrieved for 1,329 such outpatients. Patients with hyperthyroidism had shortened APTT (28.4 vs. 29.4 s; P = 0.043) and higher fibrinogen (4.1 vs. 3.8 g/l; P = 0.028) values when compared with euthyroid patients, whereas no significant differences were observed between euthyroid patients and those with hypothyroidism. Hyperthyroid patients had also an increased prevalence of shortened APTTs (<25.5 s) and higher values of fibrinogen (>4.0 g/l) than those with normal thyroid function. This study confirms that hyperthyroidism might be associated with hypercoagulability due to shortened APTT and increased fibrinogen values. Should these findings be confirmed, consideration should be given to prevent thrombotic complications in patients with hyperthyroidism.