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      The corrected QT (QTc) prolongation in hyperthyroidism and the association of thyroid hormone with the QTc interval

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          Abstract

          Purpose

          Ventricular repolarization is assessed using the QT interval corrected by the heart rate (QTc) via an electrocardiogram (ECG). Prolonged QTc is associated with an increased risk of arrhythmias and cardiac mortality. As there have been few reports regarding the effects of hyperthyroidism on ventricular repolarization, we studied the association between serum free thyroxine (free T4 [fT4]) and thyroid stimulating hormone (TSH) levels and the QTc interval.

          Methods

          Thirty-eight patients with hyperthyroidism (<30 years old) were included, and we used their clinical records and available ECGs (between August 2003 and August 2011) to evaluate the association between their fT4 and TSH levels and their QTc interval. In addition, we studied the ECGs of 72 age-matched patients with no hyperthyroidism (control group) and compared their data with that from the patients group.

          Results

          The QTc duration in patients with hyperthyroidism was significantly prolonged compared to that in the control subjects ( P<0.001). In addition, the number of hyperthyroid patients with abnormal prolonged QTc was significantly higher than that in the control group ( P<0.001). Among the patients with hyperthyroidism, patients with prolonged QTc and borderline QTc had higher fT4 levels and there was positive correlation between their fT4 levels and their QTc interval ( P<0.05). However, no correlation was observed between their TSH levels and their QTc interval.

          Conclusion

          We report that hyperthyroidism is associated with QTc prolongation. The correlation between the fT4 levels and the QTc interval suggests that thyroid status is associated with QTc values and the risk of cardiac mortality.

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          Most cited references 19

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          QT interval prolongation as predictor of sudden death in patients with myocardial infarction.

          Fifty-five patients with recent myocardial infarction and 55 healthy controls, matched for age, sex, race, height, weight, education and job, had an electrocardiogram taken every two months for seven years. Twenty-eight patients and one control had a sudden cardiac death. The QTc (mean of all values recorded) was found prolonged in one control (2%), five of 27 surviving patients (18%) and in 16 of 28 patients who had sudden death (57%). The difference between surviving and sudden death patients is significant (P less than 0.01). It is interesting that the only control with a long QT was the one when died suddenly of myocardial infarction. Among patients with previous myocardial infarction a prolonged QTc constitutes a 2.16 times greater risk for sudden death. We conclude that a constant prolongation of QTc in patients with myocardial infarction may help, with other risk factors, in defining a subgroup at higher risk for sudden death.
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            QTc: how long is too long?

            Congenital long QT syndrome (LQTS) affects an estimated 1 in 2500 people and typically presents with syncope, seizures or sudden death. Whereas someone exhibiting marked prolongation of the QT interval with QTc exceeding 500 ms who was just externally defibrillated from torsades de pointes while swimming poses negligible diagnostic challenge as to the unequivocal probability of LQTS, the certainty is considerably less for the otherwise asymptomatic person who happens to host a QTc value coined "borderline" (QTc > or = 440 ms). Although a normal QT interval imparts a much lower risk of life-threatening events, it does not preclude a patient from nevertheless harbouring a potentially lethal LQTS-causing genetic mutation. Indeed, genetic testing exerts significant diagnostic, prognostic and therapeutic implications. However, the 12-lead ECG remains the universal initial diagnostic test in the evaluation of LQTS and is subject to miscalculation, misinterpretation and mishandling. This review discusses the components of accurate QTc measurement and diagnosis, re-examines what is known about factors affecting QT interval measurement, and clarifies current recommendations regarding diagnosis of so-called "borderline" QT interval prolongation. The current guideline recommendations for the athlete with LQTS are also summarised.
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              Thyrotoxicosis and the heart.

              This review examines the molecular mechanisms by which thyroid hormone affects the cardiovascular system in naturally occurring thyroid disease states. The potential utility of thyroid hormone therapy in the management of patients with various forms of cardiovascular disease is also discussed.
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                Author and article information

                Journal
                Korean J Pediatr
                Korean J Pediatr
                KJP
                Korean Journal of Pediatrics
                The Korean Pediatric Society
                1738-1061
                2092-7258
                July 2015
                22 July 2015
                : 58
                : 7
                : 263-266
                Affiliations
                Department of Pediatrics, Hallym University College of Medicine, Chuncheon, Korea.
                Author notes
                Corresponding author: Phil Soo Oh, MD. Department of Pediatrics, Hallym University College of Medicine, 1 Hallimdaehak-gil, Chuncheon 200-702, Korea. Tel: +82-33-240-5169, Fax: +82-33-255-6244, ohphilia@ 123456unitel.co.kr
                Article
                10.3345/kjp.2015.58.7.263
                4543186
                Copyright © 2015 by The Korean Pediatric Society

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                Categories
                Original Article

                Pediatrics

                hyperthyroidism, long qt syndrome, cardiac arrhythmias

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