
 
 
 Thyroid dysfunction has a strong association with cardiac arrhythmias. However, subclinical hypothyroidism (SH) is often neglected in studies. SH is characterized by increased values of TSH and normal values of T3/T4. Recent studies indicate that high TSH levels are related to an increased incidence of cardiac electrical abnormalities, especially atrial fibrillation (AF).
 
 
 
 The present work aims to investigate direct effects of TSH on ion channel remodelling in cardiomyocytes and a possible causative role in the increased occurrence of arrhythmia in SH.
 
 
 
 A retrospective monocentric study analysed medical records, ECGs and echocardiographies of 3134 patients between 2007 to 2020. Inclusion criteria comprised patient age ≥18 years, normal fT4, TSH values 4–75 mU/l and available ECGs. The patient cohort was divided into two groups based on current treatment recommendations for SH (TSH level 4–9.9 mU/l; 10–75 mU/l).
 Direct effects on ion channel remodelling were analysed in HL-1 and neonatal cardiomyocytes with confirmed expression of TSH receptors. TSH concentrations of 15, 30, and 60 mU/l simulated different degrees of hypothyroidism. RT-qPCR analyses, western blots and immunofluorescence stainings were performed after incubation with respective TSH concentrations for 24h. In vitro video analyses of automatism in NRCM before and after TSH incubation were performed.
 Statistical comparisons were calculated using student's t-tests and the chi-square test. In addition, a multivariate analysis of factors affecting the occurrence of AF in patients with SH was carried out.
 
 
 
 Elevated TSH levels were associated with higher prevalence of AF. In the group of patients with a TSH value of 10–75 mU/l, AF was observed in 42.8% of cases compared to 31.7% in patients with lower TSH levels (p<0.0001). In vitro, TSH application to cardiomyocytes led to significant changes in mRNA and protein expression of key ion channels and regulatory proteins. Incubation of HL-1 cells with 15 mU/l TSH showed a decrease in Kcne1 protein, while 60 mU/l TSH caused an upregulation of K2P3.1 (TASK-1), Kv7.1, and KCa2.1 expression. To further elucidate the TSH signaling cascade in cardiomyocytes, we investigated TSH effects on PKAα, CaMKIIδ and CaMKIIγ. PKAα was upregulated after 24h incubation with 60 mU/l TSH and exhibited an increase of the phosphorylated state. Finally, neonatal cardiomyocytes showed an increased beating rate after incubation with TSH.
 
 
 
 The prevalence of atrial fibrillation is significantly increased in patients with SH. Our findings further reveal a direct effect of TSH on ion channel expression in cardiomyocytes as potential proarrhythmic mechanism. TSH induced changes in the expression of ion channels that determine action potential duration. Subclinical hypothyroidism contributes to the development of atrial fibrillation. Underlying mechanisms and therapeutic implications require further research.
 
 
 
 Type of funding sources: Public Institution(s). Main funding source(s): Heidelberg University Medical School - Olympia Morata


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Elevated thyroid-stimulating hormone (TSH) is associated with atrial fibrillation and directly modulates electrophysiological properties of atrial cardiomyocytes

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Elevated thyroid-stimulating hormone (TSH) is associated with atrial fibrillation and directly modulates electrophysiological properties of atrial cardiomyocytes