Background: In a recent study of patients using vitamin K antagonists, those with low free thyroxin (FT₄) levels within the normal range had a 3- to 5-fold increased risk of major bleeding. We tested the hypothesis that low levels of preoperative FT₄ within the reference range are associated with an increased risk of major bleeding during and after bariatric surgery. Methods: The charts of 2,872 consecutive patients undergoing bariatric surgery were retrospectively screened for bleeding episodes. Patients with major bleeding until 1 month after surgery were compared to randomly selected control patients without bleeding, in a ratio of 1:4. We evaluated the association between preoperative FT₄ levels and the risk of major bleeding by logistic regression. Results: Seventy-two cases(2.5%) withmajor bleeding were identified and 288 controls were selected. The median plasma level of FT₄ was 13 pmol/l (interquartile range: 12-14) in the cases as well as in the controls. No clear effect was observed of low levels of FT₄ on the risk of major bleeding: odds ratio 1.48 (95% CI: 0.46-4.80) for patients with an FT₄ level <11 pmol/l, 1.03 (0.49-2.18) for patients with an FT₄ level <12 pmol/l, and 1.12 (0.65-1.94) for patients with an FT₄ level <13 pmol/l as compared to patients with FT₄ values greater than or equal to these cutoff levels. Interpretation: We did not observe an increased risk of major bleeding with low levels of FT₄ in patients undergoing bariatric surgery.

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