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Johannes Wolfgang Christian Dietrich, Patrick Müller, Fabian Schiedat, Markus Schlömicher, Justus Strauch, Apostolos Chatzitomaris, Horst Harald Klein, Andreas Mügge, Josef Köhrle, Eddy Rijntjes, Ina Lehmphul

• \(\textit {Background:}\) Although hyperthyroidism predisposes to atrial fibrillation, previous trials have suggested decreased triiodothyronine (\(T_{3}\)) concentrations to be associated with postoperative atrial fibrillation (POAF). Therapy with thyroid hormones (TH), however, did not reduce the risk of POAF. This study reevaluates the relation between thyroid hormone status, atrial electromechanical function and POAF. \(\textit {Methods:}\) Thirty-nine patients with sinus rhythm and no history of atrial fibrillation or thyroid disease undergoing cardiac surgery were prospectively enrolled. Serum concentrations of thyrotropin, free (F) and total (T) thyroxine (\(T_{4}\)) and \(T_{3}\), reverse (r)\(T_{3}\), 3-iodothyronamine (3-\(T_{1}\)AM) and 3,5-diiodothyronine (3,5-\(T_{2}\)) were measured preoperatively, complemented by evaluation of echocardiographic and electrophysiological parameters of cardiac function. Holter-ECG and telemetry were used to screen for POAF for 10 days following cardiac surgery. \(\textit {Results:}\) Seven of 17 patients who developed POAF demonstrated nonthyroidal illness syndrome (NTIS; defined as low \(T_{3}\) and/or low \(T_{4}\) syndrome), compared to 2 of 22 (p < 0.05) patients who maintained sinus rhythm. In patients with POAF, serum \(FT_{3}\) concentrations were significantly decreased, but still within their reference ranges. 3,5-\(T_{2}\) concentrations directly correlated with r\(T_{3}\) concentrations and inversely correlated with \(FT_{3}\) concentrations. Furthermore, 3,5-\(T_{2}\) concentrations were significantly elevated in patients with NTIS and in subjects who eventually developed POAF. In multivariable logistic regression \(FT_{3}\), 3,5-\(T_{2}\), total atrial conduction time, left atrial volume index and Fas ligand were independent predictors of POAF. \(\textit {Conclusion:}\) This study confirms reduced \(FT_{3}\) concentrations in patients with POAF and is the first to report on elevated 3,5-\(T_{2}\) concentrations in cardiac NTIS. The pathogenesis of NTIS therefore seems to involve more differentiated allostatic mechanisms.