Although many case reports and observational studies have reported a correlation between iron deficiency anaemia (IDA) and thrombotic events, the mechanism for this is poorly understood. To evaluate this, we examined the change in coagulability in patients receiving treatment for IDA. Adult patients with IDA were recruited for this study and treated with intravenous iron. The change in coagulability was assessed by thrombin generation using the calibrated automated thrombogram method. The change in factor VIII (FVIII) activity was later examined as a possible link. Forty-eight participants received intravenous iron and were included in this study. After treatment with intravenous iron, endogenous thrombin potential and peak height decreased in IDA patients by a mean of 122·4 nmol/l/min (95% confidence interval [CI]: 17·9-227, P = 0·023) and 51·9 (95% CI: 26·6-77·2, P < 0·001) respectively. Time to peak (peak time) increased by a mean of 23·6 s (95% CI: 5·4-41·9, P = 0·012). FVIII activity was reduced by a mean of 9·6% (95% CI: 2·54-16·7, P = 0·009). In conclusion, treating IDA reduces the blood's coagulability, as evidenced by the change in thrombin generation and FVIII activity levels. No correlation was found between the degree of iron deficiency correction and thrombogram parameters. © 2019 British Society for Haematology and John Wiley & Sons Ltd.