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.

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