Clinical significance of serum TSH in euthyroid patients with paroxysmal atrial fibrillation

Effects of thyroid dysfunction and thyroid stimulating hormone levels on the Atrial fibrillation risks: a systematic review and dose-response meta-analysis from cohort studies

OBJECTIVE

To explore the relationship between thyroid dysfunction, thyroid stimulating hormone (TSH) levels and risks of atrial fibrillation (AF) in studies as well as to conduct a dose-response meta-analysis on the correlation between TSH levels and AF risks.

METHODS

13 studies from five databases with 649,293 subjects (mean age 65.1 years) were included. The dose-response meta-analysis was conducted by comparing the Risk Ratios (RRs) and 95% Confidence Intervals (CI) for incident AF associated with different levels of TSH (vs TSH=0 mU/L) across studies. Data were collected until Oct 25, 2021.

RESULTS

Subclinical hyperthyroidism, subclinical hypothyroidism, clinical hyperthyroidism were associated with the increased risk of AF respectively (RR=1.70, 95% CI: 1.11-2.62; RR=1.23, 95% CI:1.05-1.44; RR=2.35, 95% CI:1.07-5.16), while clinical hypothyroidism was not found associated with the significantly increased risk of AF (RR=1.20, 95% CI: 0.72-1.99). A non-linear relationship was observed in two models (crude model: P_nonlinear<0.001; adjusted model: P_nonlinear =0.0391) between the TSH concentrations and the risks of AF.

CONCLUSIONS

Our study indicated that subclinical hyperthyroidism, subclinical hypothyroidism, clinical hyperthyroidism were associated with the risk of AF, and the results for TSH levels and AF risk were mixed which showed a U-shaped relationship.

The mechanisms of atrial fibrillation in hyperthyroidism

Atrial fibrillation (AF) is a complex condition with several possible contributing factors. The rapid and irregular heartbeat produced by AF increases the risk of blood clot formation inside the heart. These clots may eventually become dislodged, causing embolism, stroke and other disorders. AF occurs in up to 15% of patients with hyperthyroidism compared to 4% of people in the general population and is more common in men and in patients with triiodothyronine (T₃) toxicosis. The incidence of AF increases with advancing age. Also, subclinical hyperthyroidism is a risk factor associated with a 3-fold increase in development of AF. Thyrotoxicosis exerts marked influences on electrical impulse generation (chronotropic effect) and conduction (dromotropic effect). Several potential mechanisms could be invoked for the effect of thyroid hormones on AF risk, including elevation of left atrial pressure secondary to increased left ventricular mass and impaired ventricular relaxation, ischemia resulting from increased resting heart rate, and increased atrial eopic activity. Reentry has been postulated as one of the main mechanisms leading to AF. AF is more likely if effective refractory periods are short and conduction is slow. Hyperthyroidism is associated with shortening of action potential duration which may also contribute to AF.

Simple electrocardiographic markers for the prediction of paroxysmal atrial fibrillation in hyperthyroidism

BACKGROUND

Hyperthyroidism is a major cause of paroxysmal atrial fibrillation (AF). The purpose of this study was to evaluate the predictors of AF in the patients with clinical and subclinical hyperthyroidism.

METHODS AND RESULTS

The study population consisted of four groups: group I (57 euthyroid healthy persons), group II (33 patients with subclinical hyperthyroidism), group III (69 patients with overt hyperthyroidism) and group IV (31 patients with overt hyperthyroidism and documented paroxysmal AF). The maximum P wave duration (P maximum) in group IV (114 +/- 8 ms) was significantly higher than group I (102 +/- 7 ms, p < 0.001), group II (106 +/- 7 ms, p < 0.001) and group III (108 +/- 9 ms, p0.005). The P wave dispersion (PWD) was measured as 46 +/- 9 ms in group IV and this was significantly higher than group I (29 +/- 8 ms, p < 0.001), group II (36 +/- 9 ms, p < 0.001) and grup III (38 +/- 8 ms, p = 0.001). The P maximum and PWD were higher in the patients with subclinical hyperthyroidism compared to healthy individuals. Univariate regression analysis revealed that age, P maximum and PWD, multivariate analysis showed that P maximum and PWD were significant predictors of paroxysmal AF. A PWD value of 37.5 ms separated group IV from others with a sensitivity of 90%, specificity of 85%, and positive predictive accuracy of 77%.

CONCLUSION

Simply measuring P maximum and PWD values, we could identify the patients with high risk for the development of AF and these simple ECG parameters may help in clinical judgement to determine the requirement for treatment in the patients with subclinical hyperthyroidism.