Noord, C. van

Invited commentary: Sex-steroid hormones and qt-interval duration (Article)

2011-08-15T00:00:00Z

In this issue of the Journal, Zhang et al. (Am J Epidemiol. 2011;174(4):403-411) make a substantial contribution to research in the area of hormonal influences on cardiac repolarization by demonstrating an inverse association between testosterone levels and the Bazett's adjusted QT interval (QTc) and RR-adjusted QT interval in men but not in postmenopausal women. They suggest that testosterone levels might explain the difference in QTc-interval duration between men and women and could contribute to population variability in QTc-interval duration among men. In this commentary, the gender difference and the role of testosterone in human cardiac repolarization are addressed. In addition, the gender differences in the congenital long-QT syndrome, drug-induced ventricular arrhythmias, and sudden cardiac death are discussed.

Proton pump inhibitors and the risk of overanticoagulation during acenocoumarol maintenance treatment (Article)

2011-05-01T00:00:00Z

In the Netherlands, several reports have described a potentiation of acenocoumarol-induced anticoagulation by co-medication of omeprazole or esomeprazole and competitive inhibition of CYP2C19 has been suggested as a possible mechanism for this interaction. We conducted an observational cohort study to investigate the effects of various proton pump inhibitors (PPIs) on acenocoumarol effectiveness. All 2755 subjects from the Rotterdam Study who received acenocoumarol maintenance treatment between April 1st, 1991 and September 9th, 2009 were followed for events of an international normalized ratio (INR)≥6, until death, end of treatment, or end of the study period. The Andersen-Gill extension of the Cox proportional hazards model was used to calculate risks for repeated events of overanticoagulation in relation to concomitant PPI use. The risk for overanticoagulation was most pronounced for esomeprazole (HR 1·99, 95% CI 1·55-2·55) and lansoprazole (HR 1·49, 95% CI 1·05-2·10). There was also a lower and non-significant risk increase for the other PPIs. We did not detect a modification of these results by CYP2C19*2 genotype. Caution should be paid to co-medication with esomeprazole and lansoprazole during acenocoumarol treatment and possibly also with other PPIs.

Non-cardiovascular drugs that inhibit hERG-encoded potassium channels and risk of sudden cardiac death (Article)

2011-02-01T00:00:00Z

Background: Virtually all QTc-prolonging drugs act by blocking the human ether a go-go-related gene (hERG)-encoded potassium channels (hERG channels), whereas not all QTc-prolonging drugs are associated with an increased risk of serious cardiac arrhythmias. This study assessed whether non-cardiovascular hERG channel blockers are associated with an increased risk of sudden cardiac death (SCD) and whether hERG-channel-inhibiting capacity is an indicator of the risk of SCD. Methods and results: The risk of SCD was studied in the Integrated Primary Care Information database, a longitudinal general practice research database. A case-control study was performed, matched for age, gender and calendar time. Odds ratios were calculated with conditional logistic regression, multivariably adjusted. In addition, the hERG-channel-inhibiting capacity of the different drugs was compared, defined as the effective free therapeutic plasma concentration (ETCPunbound) divided by the concentration that inhibits 50% of the potassium channels (IC50), with the risk of SCD. 1424 cases of SCD and 14 443 controls were identified. Current use of hERG channel blockers was associated with an increased risk of SCD. The risk of SCD was significantly increased in users of antipsychotic drugs. Patients using hERG channel blockers with a high ETCPunbound/IC50 ratio (≥0.033) had a higher risk of SCD than patients using drugs with a low ETCPunbound/IC50 ratio (<0.033). Conclusions: The current use of hERG channel blockers was associated with an increased risk of SCD in the general population. In addition, drugs with a high hERG-channel-inhibiting capacity had a higher risk of SCD than drugs with a low hERG-channel-inhibiting capacity.

CYP2C19*2 polymorphism is associated with increased survival in breast cancer patients using tamoxifen (Article)

2010-10-01T00:00:00Z

Abstract AIMS: Variant alleles of the CYP2C19 gene were recently associated with survival in breast cancer patients on tamoxifen therapy. CYP2C19 is one of the enzymes involved in the metabolism of tamoxifen into active metabolites. We investigated the hypothesis that CYP2C19*2 and *3 variants, known for their lack of enzyme activity, are associated with an increased breast cancer mortality rate in patients using tamoxifen. MATERIALS & METHODS: In the prospective population based Rotterdam study, the association between CYP2C19*2 carriers and breast cancer mortality was studied among 80 incident users of tamoxifen. Survival was analyzed with life tables and Cox regression analysis, with drug exposure as a time-dependent variable. Adjustments were made for calendar time, average tamoxifen dose, age, the indication for tamoxifen, CYP2D6 genotype and concomitant use of CYP2C19 inhibitors or inducers. RESULTS: In patients on tamoxifen, CYP2C19*2 carriers were associated with a significantly longer breast cancer survival rate than patients with the wild-type (hazard ratio 0.26, 95%CI: 0.08-0.87). CONCLUSION: This study suggests that CYP2C19 genotype may possibly be a predictive factor for survival in breast cancer patients using tamoxifen.

Drug- and non-drug-associated QT interval prolongation (Article)

2010-07-01T00:00:00Z

Sudden cardiac death is among the most common causes of cardiovascular death in developed countries. The majority of sudden cardiac deaths are caused by acute ventricular arrhythmia following repolarization disturbances. An important risk factor for repolarization disturbances is use of QT prolonging drugs, probably partly explained by gene-drug interactions. In this review, we will summarize QT interval physiology, known risk factors for QT prolongation, including drugs and the contribution of pharmacogenetics. The long QT syndrome can be congenital or acquired. The congenital long QT syndrome is caused by mutations in ion channel subunits or regulatory protein coding genes and is a rare monogenic disorder with a mendelian pattern of inheritance. Apart from that, several common genetic variants that are associated with QT interval duration have been identified. Acquired QT prolongation is more prevalent than the congenital form. Several risk factors have been identified with use of QT prolonging drugs as the most frequent cause. Most drugs that prolong the QT interval act by blocking hERG-encoded potassium channels, although some drugs mainly modify sodium channels. Both pharmacodynamic as well as pharmacokinetic mechanisms may be responsible for QT prolongation. Pharmacokinetic interactions often involve drugs that are metabolized by cytochrome P450 enzymes. Pharmacodynamic gene-drug interactions are due to genetic variants that potentiate the QT prolonging effect of drugs. QT prolongation, often due to use of QT prolonging drugs, is a major public health issue. Recently, common genetic variants associated with QT prolongation have been identified. Few pharmacogenetic studies have been performed to establish the genetic background of acquired QT prolongation but additional studies in this newly developing field are warranted.

Common variants in KCNN3 are associated with lone atrial fibrillation (Article)

2010-03-01T00:00:00Z

Atrial fibrillation (AF) is the most common sustained arrhythmia. Previous studies have identified several genetic loci associated with typical AF. We sought to identify common genetic variants underlying lone AF. This condition affects a subset of individuals without overt heart disease and with an increased heritability of AF. We report a meta-analysis of genome-wide association studies conducted using 1,335 individuals with lone AF (cases) and 12,844 unaffected individuals (referents). Cases were obtained from the German AF Network, Heart and Vascular Health Study, the Atherosclerosis Risk in Communities Study, the Cleveland Clinic and Massachusetts General Hospital. We identified an association on chromosome 1q21 to lone AF (rs13376333, adjusted odds ratio = 1.56; P = 6.3 × 10 12), and we replicated this association in two independent cohorts with lone AF (overall combined odds ratio = 1.52, 95% CI 1.40-1.64; P = 1.83 × 10 21). rs13376333 is intronic to KCNN3, which encodes a potassium channel protein involved in atrial repolarization.

Serum glucose and insulin are associated with QTc and RR intervals in nondiabetic elderly (Article)

2010-02-01T00:00:00Z

Aims: To study whether nondiabetic persons with impaired fasting serum glucose and hyperinsulinemia have QTc/QT interval prolongation and RR interval shortening in the electrocardiogram (ECG), and whether these were associated with an increased risk of sudden cardiac death. Methods: This study consisted of two analyses. First, a cross-sectional analysis was used as part of the population-based Rotterdam Study including 1050 men and 1520 women (≥55 years) without diabetes mellitus. Participants in round 3 of the Rotterdam Study for whom an ECG and fasting serum glucose and fasting insulin measurements were available were eligible for the study. Participants using digoxin or QTc-prolonging drugs and participants with left ventricular hypertrophy and left and right bundle branch block were excluded. The endpoints of the study were the lengths of the QTc, QT, and RR intervals. The associations were examined by means of linear regression analysis. Secondly, in all 6020 participants of the Rotterdam Study with an ECG, the associations between the QTc, QT, and RR intervals and sudden cardiac death were examined by means of Cox regression analysis. Results: Overall, there was a significant association between impaired fasting serum glucose and the QTc interval with an increase of 2.6 ms (95% confidence interval (CI): 0.3; 5.0) in those with fasting glucose >6 mmol/l. Hyperinsulinemia was also associated with QTc prolongation (3.0 ms (0.8; 5.3)) in those with fasting insulin ≥100 pmol/l. Impaired fasting glucose (IFG) and hyperinsulinemia were significantly associated with a decrease of the RR interval (-33.7 ms (-48.8;-18.6) and -44.4 ms (-58.7; -30.0) respectively). Participants in the fourth quartile of the QTc and QT intervals had a significantly increased risk of sudden cardiac death compared to participants in the first quartile (hazard ratio (HR) 2.87 (95% CI: 2.02-4.06); HR 3.05 (1.99-4.67) respectively). Furthermore, there was a significant inverse association between the fourth quartile of the RR interval compared to the first quartile and the risk of sudden cardiac death (HR 0.49 (0.34-0.80)). Conclusion: In this population-based study, we demonstrated that IFG and hyperinsulinemia are associated with a significantly increased QTc interval and with significant shortening of the RR interval, the latter probably due to an increased sympathetic activity. In addition, we demonstrated that both a prolonged QTc interval and a shortened RR interval are associated with an increased risk of sudden cardiac death.

Genome-wide association study of PR interval (Article)

2010-02-01T00:00:00Z

The electrocardiographic PR interval (or PQ interval) reflects atrial and atrioventricular nodal conduction, disturbances of which increase risk of atrial fibrillation. We report a meta-analysis of genome-wide association studies for PR interval from seven population-based European studies in the CHARGE Consortium: AGES, ARIC, CHS, FHS, KORA, Rotterdam Study, and SardiNIA (N = 28,517). We identified nine loci associated with PR interval at P 5 × 10 8. At the 3p22.2 locus, we observed two independent associations in voltage-gated sodium channel genes, SCN10A and SCN5A. Six of the loci were near cardiac developmental genes, including CAV1-CAV2, NKX2-5 (CSX1), SOX5, WNT11, MEIS1, and TBX5-TBX3, providing pathophysiologically interesting candidate genes. Five of the loci, SCN5A, SCN10A, NKX2-5, CAV1-CAV2, and SOX5, were also associated with atrial fibrillation (N = 5,741 cases, P 0.0056). This suggests a role for common variation in ion channel and developmental genes in atrial and atrioventricular conduction as well as in susceptibility to atrial fibrillation.

The association of serum testosterone levels and ventricular repolarization (Article)

2010-01-01T00:00:00Z

It is assumed that testosterone is an important regulator of gender-related differences in ventricular repolarization. Therefore, our aim was to study whether serum levels of testosterone are associated with QTc, QT and RR interval variation. Setting: two independent population-based cohort studies. Participants: 445 male participants (≥ 55 years) from the Rotterdam study cohort and 1,428 male participants from the study of health in Pomerania (SHIP) with an electrocardiogram who were randomly sampled for assessment of serum testosterone at baseline, after exclusion of participants with testosterone altering drugs, QTc prolonging drugs or dig(it)oxin, left ventricular hypertrophy and left and right bundle branch block. Endpoints: length of the QTc, QT and RR intervals. Analysis: linear regression model, adjusted for the two individual studies and a pooled analysis of both studies. The pooled analysis of the Rotterdam study and SHIP showed that the QTc interval gradually decreased among the tertiles (P value for trend 0.024). The third tertile of serum testosterone was associated with a lower QTc interval compared to the first tertile [-3.4 ms (-6.5; -0.3)]. However, the third tertile of serum testosterone was not associated with a lower QT interval compared to the first tertile [-0.7 ms (-3.1; 1.8)]. The RR interval gradually increased among the tertiles (P value for trend 0.002) and the third tertile of serum testosterone showed an increased RR interval compared to the first tertile [33.5 ms (12.2; 54.8)]. In the pooled analysis of two population-based studies, serum testosterone levels were not associated with the QT interval, which could be due to a lack of power. Lower QTc intervals in men with higher serum testosterone levels could be due to the association of serum testosterone with prolongation of the RR interval.

Pharmacological and Biological Risk Factors for Cardiac Arrhythmias (Doctoral Thesis)

2009-12-11T00:00:00Z

Sudden death is among the most common causes of death in developed countries. Sudden death from cardiac causes accounts for approximately 50% of all deaths from cardiovascular diseases and 20% of all deaths. The majority (80-85%) of sudden cardiac deaths are caused by acute ventricular arrhythmia. An important potential cause of ventricular arrhythmia is prolongation of ventricular repolarization, for instance, as is observed in the rare and genetically-determined ´congenital long QT syndrome´. Prolongation of ventricular repolarization may result in early after depolarizations (EAD), which in turn may induce re-entry and thereby provoke Torsade de Pointes and fatal ventricular arrhythmia.

Genetic variation in NOS1AP is associated with sudden cardiac death: Evidence from the Rotterdam Study (Article)

2009-10-20T00:00:00Z

Common variation within the nitric oxide-1 synthase activator protein (NOS1AP) locus is strongly related to QT interval, a sudden cardiac death (SCD) risk factor. A recent report describes common variation in NOS1AP associated with SCD in a US population of European ancestry. The objective of the current study was to obtain additional evidence by investigating the association between NOS1AP variants and SCD in the prospective population-based Rotterdam Study. The study population consisted of 5974 European ancestry subjects, aged 55 years and older, genotyped on Illumina arrays. SCD was defined according to European Society of Cardiology guidelines. Smoking, body mass index, diabetes mellitus, hypertension, heart failure and myocardial infarction were used as covariates in Cox proportional hazard models. Results were combined with reported evidence using inverse-variance weighted meta-analysis. Two hundred and eight (109 witnessed) cases of SCD occurred during a mean follow-up of 10.4 years. Within the Rotterdam Study alone, no significant associations were observed. Upon pooling of results with existing data, we observed strengthening of existing evidence for rs16847549 (US data HR = 1.31, P = 0.0024; Rotterdam Study HR = 1.18, P = 0.16; joint HR = 1.26, P = 0.0011). When the case definition in the Rotterdam Study was restricted to witnessed SCD, association of rs16847549 with SCD became stronger (joint P = 0.00019) and additionally the association between rs12567209 and SCD gained significance (US data HR = 0.57, P = 0.0035; Rotterdam Study HR = 0.69, P = 0.23; joint HR = 0.60, P = 0.0018). In conclusion, this study provided additional evidence for association between genetic variation within NOS1AP and SCD. The mechanism by which this effect is exerted remains to be elucidated.

Population-based studies of antithyroid drugs and sudden cardiac death (Article)

2009-09-01T00:00:00Z

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT • Thyroid hormone free T4 is associated with QTc-interval prolongation, which is a risk factor for sudden cardiac death. • The association between hyperthyroidism and ventricular arrhythmias or sudden cardiac death has been reported in several case reports. WHAT THIS STUDY ADDS • We investigated in a prospective population-based cohort study and in a case-control study whether use of antithyroid drugs (as a direct cause or as an indicator of poorly controlled hyperthyroidism) is associated with an increased risk of sudden cardiac death. • Use of antithyroid drugs was associated with a threefold increased risk of sudden cardiac death. • Although this might be due to antithyroid drug use, it could be more readily explained by underlying hyperthyroidism. AIM Thyroid free T4 is associated with QTc-interval prolongation, which is a risk factor for sudden cardiac death (SCD). Hyperthyroidism has been associated with SCD in case reports, but there are no population-based studies confirming this. The aim was to investigate whether use of antithyroid drugs (as a direct cause or as an indicator of poorly controlled hyperthyroidism) is associated with an increased risk of SCD. METHODS We studied the occurrence of SCD in a two-step procedure in two different Dutch populations. First, the prospective population-based Rotterdam Study including 7898 participants (≥55 years old). Second, we used the Integrated Primary Care Information (IPCI) database, which is a longitudinal general practice research database to see whether we could replicate results from the first study. Drug use at the index date was assessed with prescription information from automated pharmacies (Rotterdam Study) or drug prescriptions from general practices (IPCI). We used a Cox proportional hazards model in a cohort analysis, adjusted for age, gender and use of QTc prolonging drugs (Rotterdam Study) and conditional logistic regression analysis in a case-control analysis, matched for age, gender, practice and calendar time and adjusted for arrhythmia and cerebrovascular ischaemia (IPCI). RESULTS In the Rotterdam Study, 375 participants developed SCD during follow-up. Current use of antithyroid drugs was associated with SCD [adjusted hazard ratio 3.9; 95% confidence interval (CI) 1.7, 8.7]. IPCI included 1424 cases with SCD and 14 443 controls. Also in IPCI, current use of antithyroid drugs was associated with SCD (adjusted odds ratio 2.9; 95% CI 1.1, 7.4). CONCLUSIONS Use of antithyroid drugs was associated with a threefold increased risk of SCD. Although this might be directly caused by antithyroid drug use, it might be more readily explained by underlying poorly controlled hyperthyroidism, since treated patients who developed SCD still had low thyroid-stimulating hormone levels shortly before death.

Variants in ZFHX3 are associated with a trial fibrillation in individuals of European ancestry (Article)

2009-08-01T00:00:00Z

We conducted meta-analyses of genome-wide association studies for atrial fibrillation (AF) in participants from five community-based cohorts. Meta-analyses of 896 prevalent (15,768 referents) and 2,517 incident (21,337 referents) AF cases identified a new locus for AF (ZFHX3, rs2106261, risk ratio RR = 1.19; P = 2.3 × 10-7). We replicated this association in an independent cohort from the German AF Network (odds ratio = 1.44; P = 1.6 × 10-11; combined RR = 1.25; combined P = 1.8 × 10-15).

Large scale replication and meta-analysis of variants on chromosome 4q25 associated with atrial fibrillation (Article)

2009-04-01T00:00:00Z

Aims: A recent genome-wide association study identified a haplotype block on chromosome 4q25 associated with atrial fibrillation (AF). We sought to replicate this association in four independent cohorts.Methods and resultsThe Framingham Heart Study and Rotterdam Study are community-based longitudinal studies. The Vanderbilt AF Registry and German AF Network (AFNet) are case-control studies. Participants with AF (n = 3508) were more likely to be male and were older than referent participants (n = 12 173; Framingham 82 ± 10 vs. 71 ± 13 years; Rotterdam 73 ± 8 vs. 69 ± 9 years; Vanderbilt 54 ± 14 vs. 57 ± 14 years; AFNet 62 ± 12 vs. 49 ± 14 years). Single nucleotide polymorphism (SNP) rs2200733 was associated with AF in all four cohorts, with odds ratios (ORs) ranging from 1.37 in Rotterdam [95 confidence interval (CI) 1.18-1.59; P = 3.1 × 10-5] to 2.52 in AFNet (95 CI 2.22-2.8; P = 1.8 × 10-49). There also was a significant association between AF and rs10033464 in Framingham (OR 1.34; 95 CI 1.03-1.75; P = 0.031) and AFNet (OR 1.30; 95 CI 1.13-1.51; P = 0.0002), but not Vanderbilt (OR 1.16; 95 CI 0.86-1.56; P = 0.33). A meta-analysis of the current and prior AF studies revealed an OR of 1.90 (95 CI 1.60-2.26; P = 3.3 × 10-13) for rs2200733 and of 1.36 (95 CI 1.26-1.47; P = 6.7 × 10-15) for rs10033464.ConclusionThe non-coding SNPs rs2200733 and rs10033464 are strongly associated with AF in four cohorts of European descent. These results confirm the significant relations between AF and intergenic variants on chromosome 4.

Calcium channel blockers, NOS1AP, and heart-rate-corrected QT prolongation (Article)

2009-04-01T00:00:00Z

Objectives To study whether NOS1AP single nucleotide polymorphisms (SNPs), rs10494366 T>G and rs10918594 C>G, modify the heart-rate-corrected QT (QTc) prolonging effect of calcium channel blockers. Background Common variation in the NOS1AP gene has been associated with QT interval variation in several large population samples. NOS1 is presumed to influence intracellular calcium. Methods The prospective population-based Rotterdam Study includes 16 603 ECGs from 7565 participants (≥ 55 years), after exclusion of patients with left ventricular hypertrophy, left and right bundle branch block, as well as carriers of pacemakers. The endpoint was the length of the QTc interval in calcium channel blocker users and non-users with the minor alleles compared with the major alleles (wild type). We used a repeated-measurement analysis, adjusted for all known confounders. Results Use of verapamil was associated with a significant QTc interval prolongation [6.0 ms 95% confidence interval (CI) 1.7; 10.2] compared with non-users. Furthermore, users of verapamil with the rs10494366 GG genotype showed significantly more QTc prolongation than users with the TT genotype [25.4 ms (95% CI: 5.9-44.9)] (P value for multiplicative interaction 0.0038). Users of isradipine with the GG genotype showed more QTc prolongation than users with the TT genotype [19.8 ms (95% CI: 1.9-37.7)]; however, SNP rs10494366 did not modify the effect on QTc interval on a multiplicative scale (P= 0.3563). SNP rs10918594 showed similar results. Conclusion In conclusion, we showed that the minor alleles of both NOS1AP SNPs significantly potentiate the QTc prolonging effect of verapamil. Pharmacogenetics and Genomics 19:260-266

Psychotropic drugs associated with corrected QT interval prolongation (Article)

2009-02-01T00:00:00Z

AIMS:: To study whether listed putative corrected QT (QTc)-prolonging psychotropic drugs indeed prolong the QTc interval under everyday circumstances and to evaluate whether this is a class effect or an individual drug effect, we conducted a prospective population-based cohort study. METHODS:: This study was conducted as part of the Rotterdam Study and included 3377 men and 4845 women (ĝ‰¥55 years) who had triennial electrocardiograms (ECGs). The primary end points of the study were the length of the QTc interval at each ECG, the difference in QTc interval between consecutive ECGs within one person, and the risk of an abnormally prolonged QTc interval. Drug use at the index date was obtained from automated dispensing records. The associations were examined by means of a repeated measurement analysis, adjusted for age, sex, diabetes mellitus, hypertension, myocardial infarction, heart failure, and use of class 1 QTc-prolonging drugs. RESULTS:: Of the 8222 participants, 813 participants (9.9%) developed QTc prolongation during follow-up and 492 participants (74.4% women) used psychotropic drugs at the time of an ECG. Starting tricyclic antidepressants increased the QTc interval significantly with 6.9 milliseconds (95% confidence interval [CI], 3.1-10.7 milliseconds) between consecutive ECGs in comparison with consecutive ECGs of participants not using tricyclic antidepressants, in particular starting amitriptyline (8.5 milliseconds; 95% CI, 2.8-14.2 milliseconds), maprotiline (13.9 milliseconds; 95% CI, 3.6-24.3 milliseconds), and nortriptyline (35.3 milliseconds; 95% CI, 8.0-62.6 milliseconds). Starting lithium also increased the QTc interval significantly (18.6 milliseconds; 95% CI, 4.8-32.4 milliseconds). CONCLUSIONS:: In this population-based prospective cohort study, we confirmed the importance of antidepressants and antipsychotics as potential contributors to QTc prolongation. Especially, starting tricyclic antidepressant drugs (as a class) is associated with a significant intraindividual increase in the QTc interval in comparison to the change in nonusers. The tricyclic antidepressants seem to prolong the QTc interval as a class effect.

High free thyroxine levels are associated with QTc prolongation in males (Article)

2008-07-01T00:00:00Z

The literature on the effect of excess thyroid hormone on ventricular repolarization is controversial. To study whether free thyroxine (T4) and TSH are associated with QTc prolongation we conducted population-based cohort study. This study was conducted as part of the Rotterdam Study and included 365 men and 574 women aged 55 years and older with an electrocardiogram, who were randomly sampled for the assessment of thyroid status (free T4/TSH) at baseline, after exclusion of participants with hypothyroidism, use of antithyroid drugs, thyroid hormones or digoxin, left ventricular hypertrophy, and left and right bundle branch block. Endpoints were the length of the QTc interval and risk of borderline QTc prolongation. The associations were examined by means of linear and logistic regression analysis, adjusted for age and gender, diabetes mellitus, myocardial infarction, hypertension, and heart failure. Overall, there was no significant association between TSH and QTc interval (0.8 ms (95% confidence interval (Cl) -3.5, 5.2) in the first quintile compared with the fifth quintile). Subjects in the fifth quintile of free T4did not have an increased QTc interval (3.2 ms (95% Cl - 1.1, 7.6)); stratification on gender showed an increment of 10.9 ms (95% Cl 3.4, 18.3) in the fifth quintile in men and 1.1 ms (95% Cl - 4.2, 6.3) in the fifth quintile of free T4in women. When compared with subjects in the first quintile, male subjects in the fifth quintile of free T4had a significantly increased risk of a borderline QTc interval and QTc prolongation (odds ratio 2.40 (95% Cl 1.20, 4.80)). High levels of free T4are associated with substantial QTc prolongation in men of up to 10 ms. The fact that free T4is also associated with a significantly increased risk of borderline and prolonged QTc values with its risk of sudden cardiac death, endorses the clinical importance of our findings.