Effects of thyroid hormones on cardiac structure: a tissue characterization study in patients with thyroid disorders before and after treatment

Deciphering the roles of triiodothyronine (T3) and thyroid-stimulating hormone (TSH) on cardiac electrical remodeling in clinical and experimental hypothyroidism

Hypothyroidism is the most frequent endocrine pathology. Although clinical or overt hypothyroidism has been traditionally associated to low T3 / T4 and high thyrotropin (TSH) circulating levels, other forms exist such as subclinical hypothyroidism, characterized by normal blood T3 / T4 and high TSH. In its different forms is estimated to affect approximately 10% of the population, especially women, in a 5:1 ratio with respect to men. Among its consequences are alterations in cardiac electrical activity, especially in the repolarization phase, which is accompanied by an increased susceptibility to cardiac arrhythmias. Although these alterations have traditionally been attributed to thyroid hormone deficiency, recent studies, both clinical trials and experimental models, demonstrate a fundamental role of TSH in cardiac electrical remodeling. Thus, both metabolic thyroid hormones and TSH regulate cardiac ion channel expression in many and varied ways. This means that the different combinations of hormones that predominate in different types of hypothyroidism (overt, subclinic, primary, central) can generate different forms of cardiac electrical remodeling. These new findings are raising the relevant question of whether serum TSH reference ranges should be redefined.

Low triiodothyronine is associated with elevation of N-terminal pro-brain natriuretic peptide (NT-proBNP) and mortality in dialysis patients

BACKGROUND

Low thyroid hormone (TH) levels and myocardial damage are common in dialysis patients and are associated with mortality. However, little is known about the role of THs on myocardial damage as has been described in primary thyroid diseases. The aim of this study was to explore the potential relationship between low total triiodothyronine (total T3) and biomarkers of myocardial damage and the effect of their interaction on mortality, to ascertain if cardiovascular damage is the link between low THs and the risk of death in dialysis patients with CKD.

MATERIAL AND METHODS

TH plasma levels, nutritional markers, inflammation and myocardial damage were studied in 296 patients undergoing peritoneal dialysis or haemodialysis, who were followed up for 16 months to ascertain the association between biochemical variables and mortality.

RESULTS

Low total T3 levels were found in 45% of patients, which was inversely correlated with C-reactive protein (CRP) and NT-proBNP, and directly correlated with albumin and transferrin. Diabetes, CRP and total T3 were risk factors for all-cause mortality, and CRP, NT-proBNP and total T3 for cardiovascular mortality.

CONCLUSIONS

Low total T3 levels are common in dialysis patients and are associated with inflammation, malnutrition and myocardial damage. The latter may be the link between low THs and all-cause and cardiovascular mortality.

Thyroid stimulating hormone directly modulates cardiac electrical activity

BACKGROUND

The electrocardiogram of hypothyroid patients shows a series of abnormalities of cardiac repolarization due to a reduction of some repolarizing K(+) currents and an increase of the L-type calcium current. Experimental and clinical works call into question the unique role of T3 and T4 in these mechanisms and correlate increased serum TSH levels with the repolarization abnormalities in patients with both subclinical and overt hypothyroidism. In this context, the aim of the present study was to investigate the direct effects of TSH upon cardiac electrical properties.

METHODS

The action potential recording and the ion channel subunits mRNA expression were obtained from left ventricle of adult rats. Additionally, the repolarizing K(+) currents and the L-type Ca(2+) current (ICa-L) were recorded in isolated rat adult ventricular myocytes by the patch-clamp technique.

RESULTS

24h exposure to TSH lengthened the action potential and slightly depolarized the resting membrane potential. TSH- receptor activation causes a reduction of the amplitude of Ito and IK1 currents caused by a reduction in channels expression. However, TSH had no effect on ICa-L, IK or IKur.

CONCLUSION

These results support the idea that some of the electrical disturbances seen in hypothyroid hearts, such as the Ito and IK1 current reduction, could be caused not by low T3 but by the elevation of circulating TSH.

Thyroxine and cardiac electrophysiology-a forgotten physiological duo?

Thyroid hormone exerts numerous effects on the cardiovascular system. Hypothyroidism can lead to various electrocardiographic and mechanical changes in the heart and blood vessels.

The potential risk for sudden cardiac death in patients with hypothyroidism have never been properly explored. However, numerous reports of various electrocardiographic changes indicative of such a risk has been published.

In this case report the occurrence of ventricular late potentials in a case of overt hypothyroidism is described and furthermore, the disappearance of these potentials with T4 treatment alone is shown.

It is concluded that the concept that undiagnosed and/or untreated hypothyroidism poses a risk for sudden cardiac death is worth exploring.

Mechanisms responsible for the altered cardiac repolarization dispersion in experimental hypothyroidism

AIMS

To identify the causes for the inhomogeneity of ventricular repolarization and increased QT dispersion in hypothyroid mice.

METHODS

We studied the effects of 5-propyl-2-thiouracil-induced hypothyroidism on the ECG, action potential (AP) and current density of the repolarizing potassium currents I(to,fast), I(to,slow), I(K,slow) and I(ss) in enzymatically isolated myocytes from three different regions of mouse heart: right ventricle (RV), epicardium of the left ventricle (Epi-LV) and interventricular septum. K(+) currents were recorded with the patch-clamp technique. Membranes from isolated ventricular myocytes were extracted by centrifugation. Kv4.2, Kv4.3, KChIP and Na/Ca exchanger proteins were visualized by Western blot.

RESULTS

The frequency or conduction velocity was not changed by hypothyroidism, but QTc was prolonged. Neither resting membrane potential nor AP amplitude was modified. The action potential duration (APD)(90) increased in the RV and Epi-LV, but not in the septum. Hypothyroid status has no effect either on I(to,slow), I(k,slow) or I(ss) in any of the regions analysed. However, I(to,fast) was significantly reduced in the Epi-LV and in the RV, whereas it was not altered in cells from the septum. Western blot analysis reveals a reduction in Kv4.2 and Kv4.3 protein levels in both the Epi-LV and the RV and an increase in Na/Ca exchanger.

CONCLUSION

From these results we suggest that the regional differences in APD lengthening, and thus in repolarization inhomogeneity, induced by experimental hypothyroidism are at least partially explained by the uneven decrease in I(to,fast) and the differences in the relative contribution of the depolarization-activated outward currents to the repolarization process.

Myocardial ultrasonic tissue characterization in patients with thyroid dysfunction

Background

Structural myocardial abnormalities have been extensively documented in hypothyroidism. Experimental studies in animal models have also shown involvement of thyroid hormones in gene expression of myocardial collagen. This study was planned to investigate the ability of ultrasonic tissue characterization, as evaluated by integrated backscatter (IBS), to early identify myocardial involvement in thyroid dysfunction.

Patients and Methods

We studied 15 patients with hyperthyroidism (HYPER), 8 patients with hypothyroidism (HYPO), 14 patients with subclinical hypothyroidism (SCH) and 19 normal (N) subjects, who had normal LV systolic function. After treatment, 10 HYPER, 6 HYPO, and 8 SCH patients were reevaluated. IBS images were obtained and analyzed in parasternal short axis (papillary muscle level) view, at left ventricular (LV) posterior wall. The following IBS variables were analyzed: 1) the corrected coefficient (CC) of IBS, obtained by dividing IBS intensity by IBS intensity measured in a rubber phantom, using the same equipment adjustments, at the same depth; 2) cardiac cyclic variation (CV) of IBS - peak-to-peak difference between maximal and minimal values of IBS during cardiac cycle; 3) cardiac cyclic variation index (CVI) of IBS - percentual relationship between the cyclic variation (CV) and the mean value of IBS intensity.

Results

CC of IBS was significantly larger (p < 0.05) in HYPER (1.57 ± 0.6) and HYPO (1.53 ± 0.3) as compared to SCH (1.32 ± 0.3) or N (1.15 ± 0.27). The CV (dB) (HYPO: 7.5 ± 2.4; SCH: 8.2 ± 3.1; HYPER: 8.2 ± 2.0) and the CVI (HYPO: 35.6 ± 19.7%; SCH: 34.7 ± 17.5%; HYPER: 37.8 ± 11.6%) were not significantly different in patients with thyroid dysfunction as compared to N (7.0 ± 2.0 and 44.5 ± 15.1%).

Conclusions

CC of IBS was able to differentiate cardiac involvement in patients with overt HYPO and HYPER who had normal LV systolic function. These early myocardial structural abnormalities were partially reversed by drug therapy in HYPER group. On the other hand, although mean IBS intensity tended to be slightly larger in patients with SCH as compared to N, this difference was not statistical significant.

Impact of resistance to thyroid hormone on the cardiovascular system in adults

BACKGROUND

The clinical manifestations of resistance to thyroid hormone (RTH) are highly variable, and the impact of RTH on the cardiovascular system has been poorly investigated.

AIM

The objective of the study was to evaluate the cardiovascular characteristics of 16 untreated and asymptomatic patients with RTH compared with 16 euthyroid healthy controls to define the cardiovascular involvement in RTH syndrome.

PATIENTS AND METHODS

Sixteen untreated and asymptomatic RTH patients (eight males; aged 33 +/- 12 yr, range 21-45 yr) and 16 controls (nine males; aged 33 +/- 5 yr, range 24-42 yr) were enrolled. Clinical data, thyroid status, and echocardiographic results were recorded.

RESULTS

Heart rate was comparable with that of controls, whereas arterial pressure was higher than controls. Mean interventricular septum diastolic thickness and mean left ventricular (LV) posterior wall diastolic thickness were significantly lower in RTH patients than controls with a consequent significant decrease of the mean LV mass and LV mass indexed by body surface area. Patients also had abnormalities of myocardial relaxation as indicated by a significant increase of peak A and consequent reduction of the early to late ratio. Finally, systemic vascular resistance was significantly higher in RTH patients than controls.

CONCLUSIONS

Our results suggest the presence of cardiovascular alterations in asymptomatic and untreated RTH patients similar to those reported in hypothyroid patients. Our strict selection likely created a bias in the inclusion of a particular type of RTH patients, who could represent a minority of patients with RTH. However, no correlation was found between the type of mutation and cardiovascular characteristics of RTH patients.

Effect of levothyroxine treatment on QT dispersion in patients with subclinical hypothyroidism

OBJECTIVE

To examine the effect of levothyroxine treatment in patients with subclinical hypothyroidism on electrocardiographic variables, especially on ventricular repolarization-related factors.

METHODS

Sixteen women (mean age, 48.2 years) with subclinical hypothyroidism were treated with levothyroxine for 16 weeks. All standard 12-lead electrocardiographic recordings were scanned and transferred to a computer, and the QT intervals were measured on 300 times magnified recordings. QT dispersion, which reflects the heterogeneity of the ventricular repolarization, was calculated by the difference between the QT maximum and the QT minimum.

RESULTS

We found that, after 16 weeks of levothyroxine treatment, the QT interval decreased from 387.2 +/- 10.8 ms to 345.6 +/- 13.0 ms (P<0.0001). The study patients exhibited a significant reduction of QT dispersion from 46.5 +/- 5.3 ms to 30.7 +/- 5.8 ms (P<0.0001). On linear regression analysis, a positive relationship was found between QT dispersion and logarithmic serum TSH levels (r = 0.492; P<0.0001).

CONCLUSION

We conclude that serum TSH concentration has a role in ventricular inhomogeneity and, therefore, that subclinical hypothyroidism may predispose to ventricular arrhythmias. A large-scale, multicenter, randomized trial should be undertaken to address the benefit-to-risk ratio of levothyroxine treatment on cardiac inhomogeneity in patients with subclinical hypothyroidism.

Changes in heart rate variability and QT dispersion in patients with overt hypothyroidism

OBJECTIVE

The aim of the present study was to evaluate the effect of clinical hypothyroidism on cardiovascular autonomic function and ventricular repolarization.

DESIGN AND METHODS

We studied 31 patients (22 females and 9 males; mean age 53.6+/-11.8 years) with overt hypothyroidism (TSH=56.2+/-14.7 microU/ml, low free thyroxine (T4), free tri-iodothyronine (T3)) and 31 euthyroid controls, to investigate the dispersion of the QT interval in electrocardiogram (ECG) (an index of inhomogeneity of repolarization) and heart rate variability (HRV; a measure of cardiac autonomic modulation). The hypothyroid patients and controls underwent a full medical examination, standard 12-lead ECG, and 24-h ambulatory ECG monitoring. The hypothyroid patients were re-examined after 6 months of treatment with L-T4.

RESULTS

Patients with hypothyroidism showed higher QT dispersion and lower HRV measures than controls (P<0.01 or P<0.001). In hypothyroid patients, standard deviation of all R-R intervals was inversely related (by simple regression) to serum (log)TSH levels (r=-0.47, P=0.008), while QT dispersion (r=0.50, P=0.004) and QTc dispersion (r=0.46, P=0.008) were directly related to (log)TSH. Parameters of HRV improved after 6 months of L-T4 treatment, with the correction of hypothyroidism, becoming comparable with those of the control subjects, whereas the QT and QTc dispersion results were found to be only partially restored, remaining higher than the controls.

CONCLUSIONS

The results of the study demonstrate that hypothyroidism is associated with a decreased sympatho-vagal modulation of the heart rate and with an increased inhomogeneity of ventricular recovery times. The assessment of HRV and QT dispersion in patients with overt hypothyroidism may represent a useful tool in monitoring the cardiovascular risks.