Baseline Left Atrial Volume Index Is Associated With Atrial Fibrillation Documented at Follow‐Up After Achievement of Euthyroidism in Patients With Hyperthyroidism: An Exploratory Prospective Cohort Study

Background Atrial fibrillation (AF) is a common cardiovascular manifestation of hyperthyroidism. Restoration of euthyroidism frequently improves rhythm, but AF may remain detectable after thyroid normalization. We evaluated whether left atrial volume index (LAVI), measured during thyrotoxicosis, was associated with AF documented after biochemical euthyroidism. Methods We conducted an exploratory prospective longitudinal cohort study of 150 consecutive adults with newly diagnosed overt hyperthyroidism at a single tertiary hospital in Myanmar. Baseline clinical assessment and transthoracic echocardiography were performed during thyrotoxicosis. Participants were followed to biochemical euthyroidism and then assessed approximately 6 months later with repeat echocardiography and 24‐h Holter monitoring. The primary association analysis was restricted to the 24 participants with AF at baseline. Baseline LAVI was modeled continuously using Firth penalized logistic regression. A baseline LAVI threshold of > 34 mL/m 2 was evaluated only as an internally derived exploratory threshold. Results Twenty‐four participants (16.0%) had AF at baseline. At follow‐up, AF was documented in 11 of 24 participants (45.8%), while AF was not detected in 13 (54.2%). Baseline LAVI was higher among participants with AF documented at follow‐up than among those without AF detected (44.2 ± 6.1 vs. 32.5 ± 4.0 mL/m 2 ; p < 0.001). In the exploratory Firth model, each 5‐mL/m 2 increase in baseline LAVI was associated with higher odds of AF documented at follow‐up (adjusted OR 2.15, 95% CI 1.20–3.85; p = 0.01). The internally derived > 34 mL/m 2 threshold had an apparent AUC of 0.86 (95% CI 0.73–0.99), sensitivity of 81.8%, and specificity of 76.9%. All 150 participants achieved biochemical euthyroidism; the overall median time from treatment initiation to euthyroidism was 3.0 months (IQR 2.0–4.0; range 1.5–6.2). Conclusions In this exploratory cohort, higher baseline LAVI was associated with AF documented 6 months after achievement of euthyroidism among patients who presented with hyperthyroidism‐associated AF. LAVI should be regarded as a structural phenotype of atrial remodeling rather than a direct measure of fibrosis. The > 34 mL/m 2 threshold was derived and tested in the same small cohort and requires independent external validation before any clinical implementation.