Subclinical Hypothyroidism Is Characterized by Increased QT Interval Dispersion among Women

Abstract

Objective: Increased QT interval dispersion (QTd) is an electrocardiographic parameter shown to be associated with malignant ventricular arrhythmias and sudden death, and QT dispersion corrected for heart rate (QTc) has emerged as a potentially important predictor of cardiac death. Increased QTd has been detected to be directly related to thyroid-stimulating hormone (TSH) levels in overt hypothyroidism, however not much is known about subclinical hypothyroidism (SH). This study was conducted to investigate the QTc in SH and determine the changes following normalization of TSH levels with L-thyroxine. Subjects and Methods: Fifty-eight women with naive SH due to Hashimoto’s thyroiditis, mean age 39.37 ± 10.43 years, and 54 age-, sex- and weight-matched controls with normal TSH were included after exclusion of any factor that might interfere with cardiac conductibility. Electrocardiographic measurements were performed with a magnifier and Bazett’s formula was used to calculate QTc. The patients were separated into two groups regarding basal TSH levels (subgroup A: 5 > TSH > 10 mIU/l, n = 36; subgroup B: TSH > 10 mIU/l, n = 22). L-Thyroxine 1–2 µg/kg/day was administered to subgroup B. Results: Mean QTc interval of the study group was significantly longer than that of the control group (100 ± 30 vs. 76 ± 30 ms, p = 0.000). It was also longer in subgroup A (5 > TSH > 10 mIU/l, n = 36) and subgroup B (p = 0.001, p = 0.000, respectively). In subgroup B, following normalization of serum TSH, mean post-treatment QTc measurement was similar to that of the control group (75 ± 40 vs. 76 ± 30 ms, p > 0.05). Conclusion: We detected prolonged QTc among SH cases. Prolongation remained significant for the whole group as well as the two subgroups. The differences in QTc were corrected when TSH levels of >10 mIU/l returned to normal.

Incidence and Risk of QTc Interval Prolongation among Cancer Patients Treated with Vandetanib: A Systematic Review and Meta-analysis

“…On the other hand, QTc interval prolongation has also been detected in patients with hypothyroidism and subclinical hypothyroidism (SH) [17], [32], [33], as well as in patients with high free thyroxine levels and hyperthyroidism [34]–[36]. Some individuals with thyroid cancer are more often associated with an abnormal thyroid function [17], [37], and are more prone to acquire prolonged QTc intervals.…”

Thyroid Hormone and Cardioprotection

“…A clinical study by Unal et al examining QT dispersion in women with scHypo concluded that this condition may predispose to ventricular arrhythmias (247). Bakiner et al noted similar QT findings with scHypo that were reversible with T4 treatment (11). Assessment of a large Framingham population, however, did not find a statistically significant link between scHypo and atrial fibrillation (112).…”

Structure and Function of the Circulatory System in Hypothyroid Patients

“…An increase of QT interval dispersion was also found in women with subclinical hypothyroidism. The differences of QTc (-QT dispersion corrected for heart rate) were normalized when TSH level (>10mIU/l) was lowered (Bakiner et al, 2008). The cardiomyocytes isolated from hypothyroid rats are characterized by a very long action potential duration compared with euthyroid cardiomyocytes, however application of T 3 reduces their action potential duration by 24% (Sun et al, 2000).…”