Relationship between heart rate variability and subclinical thyroid disorders of the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil)

The association between subclinical thyroid dysfunctions and autonomic modulation changes has been described by many studies with conflicting results. We aimed to analyze the association between subclinical hyperthyroidism (SCHyper), subclinical hypothyroidism (SCHypo), and heart rate variability (HRV) using the baseline from ELSA-Brasil. SCHyper and SCHypo were classified by use of medication to treat thyroid disorders, thyrotropin levels respectively above and under the reference range, and normal free thyroxine levels. For HRV, the participants underwent 10 min in supine position and the R-R intervals of the final 5 min were selected for analysis. We first used linear regression models to report crude data and then, multivariate adjustment for sociodemographic (age, sex, and race) and cardiovascular risk factors (hypertension, dyslipidemia, diabetes, smoking, body mass index, use of alcohol, and leisure physical activity) using the euthyroid group as reference. From 9270 subjects (median age, 50; interquartile range: 44–56), 8623 (93.0%) were classified as euthyroid, 136 (1.5%) as SCHyper, and 511 (5.5%) as SCHypo. Compared to euthyroid subjects, SCHyper participants presented significantly higher heart rate (68.8 vs 66.5 for euthyroidism, P=0.007) and shorter R-R intervals (871.4 vs 901.6, P=0.007). Although SCHyper was associated with lower standard deviation of NN interval (SDNN) (β: –0.070; 95% confidence interval (95%CI): –0.014 to –0.009) and low-frequency (LF) (β: –0.242, 95%CI: –0.426 to –0.058) compared to the euthyroid group, these differences lost significance after multivariate adjustment for confounders. No significant differences were found for HRV in SCHypo. No association was found between HRV and SCHyper or SCHypo compared to euthyroid subjects in this sample of apparently healthy subjects.

多项研究已探讨亚临床甲状腺功能异常与自主神经调节改变之间的关联，但所得结果存在分歧。本研究旨在借助巴西成人健康纵向研究（ELSA-Brasil）的基线数据，分析亚临床甲状腺功能亢进症（SCHyper）、亚临床甲状腺功能减退症（SCHypo）与心率变异性（HRV）之间的关联。本研究依据甲状腺疾病治疗药物使用情况、促甲状腺素水平分别高于或低于参考范围，且游离甲状腺素水平正常的标准，对亚临床甲状腺功能亢进症与亚临床甲状腺功能减退症进行分类。关于心率变异性的检测，受试者取仰卧位静息10分钟，选取最后5分钟的R-R间期进行分析。本研究首先采用线性回归模型报告未校正的粗数据，随后以甲状腺功能正常组为参照，对社会人口学因素（年龄、性别、种族）及心血管危险因素（高血压、血脂异常、糖尿病、吸烟、体质量指数、饮酒习惯、闲暇体力活动）进行多因素校正。本研究共纳入9270名受试者，年龄中位数为50岁，四分位间距为44~56岁；其中8623人（93.0%）被归类为甲状腺功能正常，136人（1.5%）为亚临床甲状腺功能亢进症患者，511人（5.5%）为亚临床甲状腺功能减退症患者。与甲状腺功能正常的受试者相比，亚临床甲状腺功能亢进症患者的心率显著更高（68.8 vs 甲状腺功能正常组的66.5，P=0.007），R-R间期更短（871.4 vs 901.6，P=0.007）。尽管与甲状腺功能正常组相比，亚临床甲状腺功能亢进症与更低的NN间期标准差（SDNN）（β：–0.070；95%置信区间（95%CI）：–0.014~–0.009）和低频成分（LF）（β：–0.242，95%CI：–0.426~–0.058）相关，但在校正混杂因素后，这些差异不再具有统计学显著性。亚临床甲状腺功能减退症组的心率变异性未观察到显著差异。在本研究纳入的看似健康的受试者样本中，未发现心率变异性与亚临床甲状腺功能亢进症或亚临床甲状腺功能减退症之间存在关联。