Hypothyroidism renders protection against lethal ventricular arrhythmias in a conscious canine model of sudden death

New aspects of endocrine control of atrial fibrillation and possibilities for clinical translation

Hormones are potent endo-, para-, and autocrine endogenous regulators of the function of multiple organs, including the heart. Endocrine dysfunction promotes a number of cardiovascular diseases, including atrial fibrillation (AF). While the heart is a target for endocrine regulation, it is also an active endocrine organ itself, secreting a number of important bioactive hormones that convey significant endocrine effects, but also through para-/autocrine actions, actively participate in cardiac self-regulation. The hormones regulating heart-function work in concert to support myocardial performance. AF is a serious clinical problem associated with increased morbidity and mortality, mainly due to stroke and heart failure. Current therapies for AF remain inadequate. AF is characterized by altered atrial function and structure, including electrical and profibrotic remodelling in the atria and ventricles, which facilitates AF progression and hampers its treatment. Although features of this remodelling are well-established and its mechanisms are partly understood, important pathways pertinent to AF arrhythmogenesis are still unidentified. The discovery of these missing pathways has the potential to lead to therapeutic breakthroughs. Endocrine dysfunction is well-recognized to lead to AF. In this review, we discuss endocrine and cardiocrine signalling systems that directly, or as a consequence of an underlying cardiac pathology, contribute to AF pathogenesis. More specifically, we consider the roles of products from the hypothalamic-pituitary axis, the adrenal glands, adipose tissue, the renin–angiotensin system, atrial cardiomyocytes, and the thyroid gland in controlling atrial electrical and structural properties. The influence of endocrine/paracrine dysfunction on AF risk and mechanisms is evaluated and discussed. We focus on the most recent findings and reflect on the potential of translating them into clinical application.

The Role of Thyroid Diseases and their Medications in Cardiovascular Disorders: A Review of the Literature

The association between thyroid disease and cardiovascular manifestations is significant and undeniable. Previous studies have explained several aspects of the effects of thyroid hormone on the heart and cardiovascular system. Accordingly, both hyper and hypothyroidism can cause important alterations in cardiac rhythm, output and contractility as well as vascular resistance and blood pressure. Since treating the thyroid abnormality, especially in its initial stages, could lead to a significant improvement in most of its resultant cardiovascular disturbances, early suspicion and recognition of thyroid dysfunction, is necessary in patients with cardiovascular manifestations. In this in-depth review, we discuss the physiological roles as well as the effects of abnormal levels of thyroid hormones on the cardiovascular system. We also review the effects of the medications used for the treatment of hyper and hypothyroidism on cardiac function. In the end, we discuss the association between thyroid function and amiodarone, an effective and frequently-used antiarrhythmic drug, because of its well-known effects on the thyroid.

Thyroid dysfunction and electrocardiographic changes in subjects without arrhythmias: a cross-sectional study of primary healthcare subjects from Copenhagen

OBJECTIVE

The objective of the present study was to investigate associations of both overt and subclinical thyroid dysfunction with common ECG parameters in a large primary healthcare population.

DESIGN

Cross-sectional study.

SETTING AND PARTICIPANTS

The study population comprised of primary healthcare patients in Copenhagen, Denmark, who had a thyroid function test and an ECG recorded within 7 days of each other between 2001 and 2011.

DATA SOURCES

The Danish National Patient Registry was used to collect information regarding baseline characteristics and important comorbidities.

OUTCOME MEASURE AND STUDY GROUPS

Common ECG parameters were determined using Marquette 12SL software and were compared between the study groups. The study population was divided into five groups based on their thyroid status. Euthyroid subjects served as the reference group in all analyses.

RESULTS

A total of 132 707 patients (age 52±17 years; 50% female) were included. Hyperthyroidism was significantly associated with higher heart rate and prolonged QTc interval with significant interaction with age (p<0.009) and sex (p<0.001). These associations were less pronounced for patients with higher age. Subclinical hyperthyroidism was associated with higher heart rate among females, and a similar trend was observed among males. Hypothyroidism was associated with slower heart rate and shorter QTc but only in women. Moreover, longer P-wave duration, longer PR interval and low voltage were observed in patients with both subclinical and overt hypothyroidism. However, the presence of low voltage was less pronounced with higher age (p=0.001).

CONCLUSION

Both overt and subclinical thyroid disorders were associated with significant changes in important ECG parameters. Age and gender have significant impact on the association of thyroid dysfunction particularly on heart rate and QTc interval.

Thyroid hormones and electrocardiographic parameters: findings from the third national health and nutrition examination survey

Introduction

Altered thyroid status exerts a major effect on the heart. Individuals with hypo- or hyperthyroidism showed various changes in electrocardiograms. However, little is known about how variations in thyroid hormone levels within the normal range affect electrical activities of the heart in the general population.

Methods and Results

We conducted a cross-sectional analysis of 5,990 men and women from the Third National Health and Nutrition Examination Survey. Serum total T4 was measured by immunoassay and TSH was measured by chemiluminescent assay. We categorized T4 and TSH into 7 groups with cut-offs at the 5^th, 20^th, 40^th, 60^th, 80^th, and 95^th percentiles of the weighted population distribution. Electrocardiographic parameters were measured from the standard 12-lead electrocardiogram. We found a positive linear association between serum total T4 level and heart rate in men, and a U-shape association between T4 and PR interval in men and women. TSH level was positively associated with QRS interval in men, while a U-shape association between TSH and QRS was observed in women. No clear graded association between thyroid hormones and corrected QT or JT was found, except that men in the highest category of T4 levels appeared to have longer corrected QT and JT, and men in the lowest category of T4 appeared to have shorter corrected QT and JT.

Conclusions

Variation in thyroid hormone levels in the general population, even within the normal range, was associated with various ECG changes.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

The effects of L-thyroxine treatment on QT dispersion in primary hypothyroidism

Hypothyroidism has various cardiovascular manifestation and exhibits electrocardiographic change. The QT dispersion on surface ECG reflects regional variations in myocardial repolarization. The effect of L-thyroxine treatment on ECG parameters, such as QT dispersion, in patients with primary hypothyroidism were investigated. This study involved 18 patients (3 men, 15 women, ages: 48+/-18 yr) with primary hypothyroidism. All patients were checked with a standard 12-lead ECG before and after L-thyroxine treatment. Various ECG parameters were then measured twice. The mean L-thyroxine treatment duration was 22+/-2.7 months. The mean thyroid-stimulating hormone levels of patients before and after therapy were 40.2+/-29.8 microU/mL, 3.6+/-4.6 microU/mL (p<0.001) and free-T4 levels were 0.44+/-0.38 ng/dL, 1.51+/-0.39 ng/dL (p<0.001). After L-thyroxine treatment, QT interval (395+/-42 vs. 380+/-24 msec, p<0.05), QTc interval (434+/-32 vs. 417+/-23 msec, p<0.05), QT dispersion (45+/-23 vs. 30+/-13 msec, p=0.008), QTc dispersion (49+/-23 vs. 32+/-14 msec, p=0.005) significantly decreased. There were no significant changes in the PR and RR intervals, as well as the QRS duration. Our findings suggest that the thyroid hormone affects ventricular inhomogenicity, and that L-thyroxine replacement therapy may reduce malignant ventricular arrhythmia and sudden cardiac death in primary hypothyroidism.

Thyroid hormone action in the heart

The heart is a major target organ for thyroid hormone action, and marked changes occur in cardiac function in patients with hypo- or hyperthyroidism. T(3)-induced changes in cardiac function can result from direct or indirect T(3) effects. Direct effects result from T(3) action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T(3) effects, which occur independent of nuclear T(3) receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T(3) effects are mediated by the binding of T(3) to specific nuclear receptor proteins, which results in increased transcription of T(3)-responsive cardiac genes. The T(3) receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. T(3) also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum. This T(3) effect results from T(3)-induced increases in the level of the mRNA coding for the sarcoplasmic reticulum calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the sarcoplasmic reticulum.

Streptozotocin diabetes protects against arrhythmias in rat isolated hearts: role of hypothyroidism

We examined the contribution of hypothyroidism to streptozotocin diabetes-induced alterations in the arrhythmia susceptibility of ex vivo hearts to regional zero-flow ischaemia. Diabetic rats received either protamine zinc insulin (10 IU/kg/day, s.c.) or triiodothyronine (10 microg/kg/day, s.c.) for 8 weeks commencing 72 h after injection of streptozotocin (60 mg/kg, i.p.). Arrhythmias were determined in ex vivo Langendorff-perfused hearts, subjected to a 30-min main left coronary artery occlusion, followed by 30-min reperfusion. Serum free thyroxine concentrations, rectal temperature and ex vivo heart rate were significantly decreased in the 8-week diabetic group (P<0.001). These changes were prevented by administration of triiodothyronine or insulin. Ventricular fibrillation during reperfusion was abolished in hearts from diabetic rats. This protection was prevented by treatment with either triiodothyronine or insulin. Hearts from methimazole-hypothyroid rats also showed no ventricular fibrillation during reperfusion. The protection against ischaemia-reperfusion-arrhythmias observed in hearts from streptozotocin-diabetic rats may be due to diabetes-induced hypothyroidism.

Effects of thyroid hormone deficiency on electrocardiogram findings of congenitally hypothyroid neonates

Hypothyroid status is believed to cause various metabolic changes in infants. However, it is interesting that even severely hypothyroid neonates, detected during mass neonatal screening, rarely show bradycardia, hypothermia, or inactivity. To study cardiac functions of screen-detected neonates with congenital hypothyroidism (CH), we recorded the electrocardiograms (ECG) of 53 screen-detected CH neonates before levothyroxine (LT4) replacement therapy, and 15 age-matched normal neonates for controls. The 53 CH neonates were divided into two groups according to initial serum thyroid hormone levels: a mildly hypothyroid group (n = 37), serum thyroid-stimulating hormone (TSH) less than 100 microIU/mL and free thyroxine (FT4) 0.6 ng/dl or more; and a severely hypothyroid group (n = 16), TSH 100 microIU/mL or more and FT4 less than 0.6 ng/dL. TSH, FT4, and other blood chemicals were measured on an autoanalyzer (Hitachi 7170). After blood sampling, the ECG was recorded during induced sleep by oral administration of triclofos sodium syrup. ECG parameters, including HR, PR, QRS, QT time and corrected QT time (QTc) were automatically obtained, using an auto-ECG analyzing system. The following results were obtained. No CH patients showed abnormal ECG findings. There was no significant difference of the mean heart rates (HRs) between the mildly hypothyroid (147.5 +/- 16.3 beats per minute) and the control group (148.3 +/- 12.1 beats per minute). The mean HR in the severely hypothyroid group (134.0 +/- 17.9 beats per minute, p = 0.007) was significantly low compared with the normal control group. However, all values were within normal ranges. QTc in the severely hypothyroid group (0.414 +/- 0.015, p = 0.033) was significantly shorter than in the control group (0.440 +/- 0.052). No statistical differences of PR, QRS, and QT time were noted among the three groups. All ECG parameters were within normal ranges. HR positively correlated with FT4 and log (FT4), and negatively with TSH and log (TSH). From these results we conclude that the deficiency of thyroid hormones does not affect ECG findings of congenitally hypothyroid neonates. This may be consistent with the unexpectedly mild signs and symptoms of screen-detected hypothyroid neonates.

Cardiovascular and atherogenic aspects of subclinical hypothyroidism

Subclinical hypothyroidism (SH) is common, especially among elderly women. There is no clear evidence to date that SH causes clinical heart disease. However, mild thyroid gland failure, evidenced solely by elevation of the serum thyrotropin (TSH) concentration, may be associated with increased morbidity, particularly for cardiovascular disease, and subtly decreased myocardial contractility. In SH, both cardiac structures and function remain normal at rest, but impaired ventricular function as well as cardiovascular and respiratory adaptation to effort may become unmasked during exercise. These changes are reversible when euthyroidism is restored. Flow-mediated vasodilatation, a marker of endothelial function, is significantly impaired in SH, and decreased heart rate variability, a marker of autonomic activity, suggests hypofunctional abnormalities in the parasympathetic nervous system. SH does result in a small increase in low-density lipoprotein (LDL) cholesterol (C) and a decrease in high-density lipoprotein (HDL)-C, changes that enhance the risk for development of atherosclerosis and coronary artery disease (CAD). After coronary revascularization, a trend toward higher rates of chest pain, dissection, and reocclusion has been noted in SH subjects. Smoking may contribute to the high incidence of SH and may aggravate its metabolic effects. Subjects with SH with marked TSH elevation and high titers of thyroid autoantibodies are at higher risk of unnoticed progression to overt hypothyroidism. Especially women over 50 years with TSH levels greater than 10 mU/L and smoking habits have the highest risk for cardiovascular complications. The magnitude of the lipid changes and the subtle impairment of left ventricular function and cardiopulmonary exercise capacity in SH may justify use of hormone replacement. Early levothyroxine (LT4) treatment in SH may reduce the C level by an average of 8% and normalize all metabolic effects in smokers, nevertheless, in some patients, LT4 therapy may exacerbate angina pectoris or an underlying cardiac arrhythmia. Longitudinal follow-up to define the actual cardiovascular disease risk associated with SH is warranted.

Electrophysiological mechanisms by which hypothyroidism delays repolarization in guinea pig hearts

Thyroid hormone is known to exert important effects on cardiac repolarization, but the underlying mechanisms are poorly understood. We investigated the electrophysiological mechanisms of differences in repolarization between control guinea pigs and hypothyroid animals (thyroidectomy plus 5-propyl-2-thiouracil). Hypothyroidism significantly prolonged the rate-corrected Q-T interval in vivo and action potential duration (APD) of isolated ventricular myocytes. Whole cell voltage-clamp studies showed no change in current density or kinetics of L-type Ca(2+) current, inward rectifier K(+) current, or Na(+) current in hypothyroid hearts. Dofetilide-resistant current (I(Ks)) step current densities were smaller by approximately 65%, and tail current densities were reduced by 80% in myocytes from hypothyroid animals compared with controls. The ratio of delayed rectifier step current at +50 mV to tail current at -40 mV was significantly larger in hypothyroid cells for test pulses from 60- to 4,200-ms duration, reflecting a smaller I(Ks). Dofetilide-sensitive current (I(Kr)) densities were not significantly changed. I(Ks) half-activation voltage shifted to more positive voltages in hypothyroidism (29.5 +/- 2.2 vs. 21.3 +/- 2.7 mV in control, P < 0.01), whereas I(Kr) voltage dependence was unchanged. We conclude that hypothyroidism delays repolarization in the guinea pig ventricle by decreasing I(Ks), a novel and potentially important mechanism for thyroid regulation of cardiac electrophysiology.

Effects of hypothyroidism on the vulnerability to ventricular fibrillation in dogs: a comparative study with amiodarone

It has been shown that thyroid hormone has a significant effect on the heart and that suppression of thyroid function may contribute to the antiarrhythmic effect of amiodarone. The study was aimed at investigating the effects of hypothyroidism, compared with those of amiodarone, on vulnerability to ventricular fibrillation in dogs. In this study, 25 adult dogs were randomly divided into three groups: a hypothyroid group following total thyroidectomy (n = 9), an amiodarone group (n = 8, 400 mg per day, 4 weeks), and a control group (n = 8). Both amiodarone and control groups were subjected to sham surgery. Five to 8 weeks after surgery, ventricular fibrillation threshold and other electrophysiological parameters were determined. Right ventricular effective refractory period, monophasic action potential duration, and ventricular fibrillation threshold were significantly increased in both the thyroidectomized and amiodarone-treated animals. There was no significant change in monophasic action potential duration dispersion. The incidence of ventricular fibrillation during ischemia and reperfusion was significantly reduced in both treated groups compared with the sham-operated euthyroid controls. These observations suggest that hypothyroidism has a significant antifibrillatory effect in dogs. Homogeneous prolongation of repolarization and refractoriness may contribute to the antifibrillatory action of hypothyroidism.

Antiarrhythmic versus antifibrillatory actions: inference from experimental studies

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.