Heart rate variability and left atrial stiffness index by 2D speckle-tracking echocardiography in subclinical thyroid dysfunction

Abstract Background Thyroid hormones significantly influence cardiac function due to the high concentration of thyroid receptors in heart muscle tissue, particularly in the myocardium. Thyroid hormone levels affect heart rate variability, which serves as an indicator of how well the cardiac autonomic system is functioning. The walls of the left atrium (LA) contain numerous sympathetic and parasympathetic nerve cells. Our research focused on examining how thyroid disorders affect the LA phasic functions and left atrial stiffness index (LASI) using two-dimensional speckle tracking echocardiography (2D STE), also analyzing heart rate variability (HRV) in individuals with subclinical thyroid dysfunction. Methods Holter monitoring for HRV was done both in frequency and time domains. Conventional two-dimensional transthoracic echo-Doppler study, tissue Doppler imaging (TDI) and 2D STE were executed for each participant included in the study. The LA longitudinal strain and LA stiffness index (LASI) were assessed using 2D STE analysis. Results The study encompassed 155 participants in total and categorized as follows: euthyroid, subclinical hypothyroidism (SCHypo), and subclinical hyperthyroidism (SCHyper). There was a statistically significant decline in the LA reservoir function and increased LASI in the subclinical thyroid disorders, whether hypothyroidism ( p = 0.01) or hyperthyroidism ( p = 0.007). There was a significantly increased low-frequency/high-frequency ratio (LF/HF) and diminished the percentage of adjacent NN intervals differing by more than 50 ms (PNN50) in the subclinical hyperthyroid group. There was a significant correlation between thyroid-stimulating hormone (TSH) and LA reservoir function ( p = 0.001) and LASI ( p = 0.04), also a significant correlation with LF/HF, PNN50% and the square root of the mean of the sum of the squared differences between successive NN intervals (RMSSD). Conclusion Impaired LA phasic function and increased left atrial stiffness index in subclinical thyroid dysfunction. Impaired autonomic nervous system functions in subclinical thyroid dysfunction especially in subclinical hyperthyroidism.