Context. Various changes in the coagulation-fibrinolytic system have been described in patients with an excess or deficiency of thyroid hormones. The purpose of this systematic review is to summarize the effects of hyperthyroidism and hypothyroidism on these systems.

Evidence Acquisition. All published case-control or interventional cohort studies that evaluated the effects of hyperthyroidism and hypothyroidism on the coagulation-fibrinolytic system in vivo were identified by a computer-assisted search of the MEDLINE and EMBASE electronic databases. A scoring system was used to divide studies into three quality categories: high, medium, and low quality.

Evidence Synthesis. A total of 36 papers were included. Since in several papers more than one case-control study or both a case-control and intervention study were described, a total of 39 case-control studies and 24 interventional cohort studies were analysed. No high quality study was identified. Three (7.7%) case-control and 8 (33.3%) cohort studies were of medium quality. A total of 19 tests were investigated in the medium quality studies. These tests revealed a hypocoagulable state for overt hypothyroidism and a hypercoagulable state for overt hyperthyroidism.

Conclusions. This analysis confirmed that clinically overt hyperthyroidism and hypothyroidism modify the coagulation-fibrinolytic balance, indicating that thyroid hormone excess or deficit is the probable main pathophysiologic mechanism. Patients with overt hypothyroidism and overt hyperthyroidism appear to have an increased risk of bleeding and of thrombosis, respectively.