ABC | Volume 112, Nº6, June 2019

762 Original Article Rajão et al Subclinical thyroid dysfunction and arrhythmias Arq Bras Cardiol. 2019; 112(6):758-766 Table 2 – Association between Heart Rate and Subclinical Thyroid Dysfunction, ELSA-Brasil, 2008-2010 Subclinical Hypothyroidism Euthyroidism Subclinical Hyperthyroidism Prevalence N (%) OR (CI 95%) “p” Prevalence N (%) Prevalence N (%) OR (CI 95%) “p” Tachycardia > 100 bpm 413 (3.10%) 23 (3.76%) 1.04 (0.67-1.59) 0.874 385 (3.63%) 5 (2.89%) 0.79 (0.32-1.93) 0.604 > 110 bpm 344 (2.58%) 17 (2.81%) 0.91 (0.56-1.50) 0.715 323 (3.07%) 4 (2.33%) 0.75 (0.28-2.04) 0.576 Bradycardia < 60 bpm 1,964 (14.72%) 86 (12.74%) 0.80 (0.64-1.01) 0.062 1,858 (15.40%) 20 (10.64%) 0.65 (0.41-1.04) 0.074 < 50 bpm 211 (1.58%) 12 (2.00%) 1.07 (0.59-1.92) 0.830 195 (1.87%) 4 (2.33%) 1.25 (0.46-3.39) 0.667 Table 3 – Relationship between Heart Rate and TSH and FT4 Levels, ELSA-Brasil, 2008-2010 TSH FT4 Median (µU/ml) p-value* Median (ng/dl) p-value* Tachycardia (> 100 bpm) 1.63 0.004 1.20 0.021 Normal HR (60-100bpm) 1.52 1.10 Bradycardia (< 60 bpm) 1.54 0.311 1.10 0.233 TSH: thyroid-stimulating hormone; FT4: free thyroxine; HR: heart rate; *after adjustment. Table 4 – Association between Abnormalities on the ECGs and Subclinical Thyroid Dysfunction, ELSA-Brasil, 2008-2010 Subclinical Hypothyroidism (615 ECGs) Euthyroidism (11,003 ECGs) Subclinical Hyperthyroidism (177 ECGs) Prevalence N (%) OR (CI 95%) “p” Prevalence N (%) Prevalence N (%) OR (CI 95%) “p” AF/Flutter 42 (0.36%) 2 (0.33%) 0.89 (0.22-3.70) 0.878 40 (0.36%) 0 (0%) ------------------ 0.422 Persistent supraventricular rhythm 89 (0.75%) 5 (0.81%) 1.08 (0.44-2.67) 0.870 83 (0.75%) 5 (1.69%) 0.75 (0.10-5.40) 0.773 Extrasystole 94 (0.80%) 4 (0.65%) 0.82 (0.30-2.25) 0.701 87 (0.79%) 3 (1.69%) 2.16 (0.68-6.90) 0.193 Long QT interval 334 (2.83%) 15 (2.15%) 0.85 (0.50-1.43) 0.532 315 (2.53%) 4 (2.07%) 0.82 (0.30-2.21) 0.688 LV QRS 166 (1.41%) 10 (1.63%) 1.16 (0.61-2.22) 0.644 154 (1.40%) 2 (1.13%) 0.81 (0.20-3.27) 0.762 Conduction disorders 2,067 (17.52%) 101 (16.42%) 0.92 (0.74-1.14) 0.437 1,942 (17.65%) 24 (13.56%) 0.73 (0.47-1.13) 0.158 AF: atrial fibrillation; LV QRS: Low QRS voltage; ECGs: electrocardiograms. Discussion This cross-sectional analysis of 13,341 individuals of this Brazilian cohort found no association of STD with HR, rhythm alterations, or conduction disorders, which suggests a limited influence of STD on cardiac rhythm and conduction. The only association found was an unexpected lower frequency of conduction disorders among the older participants with SCHypoTh compared to euthyroid participants, and it may be due to some unknown confounding factor not estimated or controlled for. Of note, no population-based study has assessed the association between electrocardiographic abnormalities such as conduction disorders, lowQRS complex voltage, prolonged QTi, and persistent supraventricular rhythms and the presence of STD, particularly SCHypoTh. Most of the studies on this subject are case reports or relate those abnormalities to clinical, rather than subclinical, hypothyroidism. There is a paucity of studies on the prevalence of cardiac arrhythmias in the general population, without known heart disease or comorbidities, and most of those available address older individuals in developed countries. This prevalence varies according to the type of arrhythmia, age, sex, presence of structural heart disease or cardiovascular risk factors, and the diagnostic method employed. 29,30 AF was found in 0.35% of the participants in the present study, which is a similar result to those of population-based studies showing prevalence rates of 0.2% to 1.0%. 28,30-32 However, considering that the prevalence of AF can be as high as 10% in individuals aged 70 years or older, 32 the frequency of AF and flutter was low (1.32%) among older adults in this study.