Thyroid Function Affects the Risk of Stroke via Atrial Fibrillation: A Mendelian Randomization Study

Impaired Thyroid Hormone Sensitivity Increases Risk of Cardiovascular Events in Patients Undergoing Coronary Angiography

CONTEXT

Previous studies found inconsistent results on the relationship between thyroid function and cardiovascular risks.

OBJECTIVE

This study aimed to investigate the association of thyroid hormone sensitivity with the risk of major adverse cardiovascular events (MACE) and cardiovascular death in a euthyroid population undergoing coronary angiography.

METHODS

This prospective cohort study enrolled 1470 euthyroid participants who underwent coronary angiography between March and November of 2013. The participants were followed up from July to November in 2022. Thyrotroph thyroxine resistance index (TT4RI), TSH index (TSHI), and feedback quantile-based index (TFQI) were calculated to evaluate the sensitivity to thyroid hormone. Kaplan-Meier curve and multivariable Cox proportional hazard model were performed to analyze the association between thyroid hormone sensitivity and risk of MACE and cardiovascular death.

RESULTS

Among 1089 participants who completed the follow-up, 342 cases of MACE and 77 cardiovascular deaths were identified during a medium follow-up duration of 111 months. In the multivariable Cox proportional hazard model, the higher levels of TFQI (hazard ratio [HR] = 1.41; 95% CI, 1.08-1.84; P for trend = .01), TT4RI (HR = 1.40; 95% CI, 1.06-1.84; P for trend = .02) and TSHI (HR = 1.61; 95% CI, 1.22-2.13; P for trend = .001) were associated with increased risk of MACE. The higher levels of TFQI (HR = 2.21; 95% CI, 1.17-4.17; P for trend = .02) and TSHI (HR = 2.05; 95% CI; 1.08-3.91; P for trend = .03) were also associated with increased risk of cardiovascular death.

CONCLUSION

Impaired sensitivity to thyroid hormone is associated with higher risks of MACE and cardiovascular death in a euthyroid population undergoing coronary angiography.

Thyroid cancer and cardiovascular diseases: a Mendelian randomization study

Background

Multiple observational studies have shown associations between thyroid cancer (TC) and cardiovascular diseases (CVDs). However, the results were inconsistent, and the potential causal genetic relationship remains unclear.

Methods

The genetic instruments of TC and CVDs were derived from data obtained through genome-wide association studies (GWAS). We performed the two-sample Mendelian randomization(MR) methods to investigate the causality of TC on CVDs. Summary-level statistics for CVDs, including heart failure (HF), atrial fibrillation (AF), coronary artery disease (CAD), myocardial infarction (MI), ischemic stroke (IS) and venous thromboembolism (VTE). The primary method employed in this MR analysis was the Inverse Variance Weighted (IVW) approach, and four additional algorithms were used: MR-Egger, weighted median, simple mode, and weighted mode. Additionally, we assessed the reliability of the causal relationship through pleiotropy, heterogeneity and leave-one-out sensitivity analysis.

Results

In this MR analysis, we only detected causality of genetically predicted TC on HF (IVW method, odds ratio (OR) = 1.00134, 95% confidence interval (CI): 1.00023-1.00244, p = 0.017). However, There were no causal associations of TC with CAD, MI, AF, IS, and VTE.

Conclusion

Our results confirmed the causal association between TC and HF. It is crucial to closely monitor the incidence of HF in TC patients and give comprehensive clinical intervention based on conventional treatment.

Causal association of TSH with ischemic heart diseases and heart failure: A 2-sample Mendelian randomization study

Thyroid dysfunction is associated with the risk of cardiovascular disease; however, whether plasma thyroid-stimulating hormone (TSH) levels in subjects with euthyroidism affect the risk of cardiovascular disease remains unclear. This study aimed to investigate the causal association between plasma TSH levels and cardiovascular diseases, particularly ischemic heart disease and heart failure (HF). Summary statistics from the Integrative Epidemiology Unit Open genome-wide association studies Project and FinnGen consortium were used to investigate the causal relationship between plasma TSH levels and the risk of cardiovascular diseases. Two-sample Mendelian randomization analysis using inverse-variance weighting as the primary method was performed. The MR Pleiotropy RESidual Sum and Outlier and leave-one-out methods were used to ensure the robustness of our findings. Genetically determined plasma TSH levels were associated with major coronary heart disease events (OR 1.0557, 95% CI 1.0141-1.0991), all-cause HF (OR 0.9587, 95% CI 0.9231-0.9956), and HF + non-ischemic cardiomyopathy (OR 0.9318, 95% CI 0.8786-0.9882). After the Bonferroni correction, the causation described above disappeared. In the secondary analysis, genetically determined higher TSH levels were associated with a higher risk for unstable angina pectoris (OR 1.0913, 95% CI 1.0350-1.1507), but were associated with a lower risk for HF + overweight (OR 0.9265, 95% CI 0.8821-0.9731). These results were further validated using sensitivity analysis. Our findings show that increased plasma TSH levels in patients with euthyroidism may increase the risk of unstable angina pectoris but reduce the risk of HF in overweight patients. This evidence indicates that plasma TSH levels may need to be carefully controlled in specific patients.

Multi-trait analysis characterizes the genetics of thyroid function and identifies causal associations with clinical implications

To date only a fraction of the genetic footprint of thyroid function has been clarified. We report a genome-wide association study meta-analysis of thyroid function in up to 271,040 individuals of European ancestry, including reference range thyrotropin (TSH), free thyroxine (FT4), free and total triiodothyronine (T3), proxies for metabolism (T3/FT4 ratio) as well as dichotomized high and low TSH levels. We revealed 259 independent significant associations for TSH (61% novel), 85 for FT4 (67% novel), and 62 novel signals for the T3 related traits. The loci explained 14.1%, 6.0%, 9.5% and 1.1% of the total variation in TSH, FT4, total T3 and free T3 concentrations, respectively. Genetic correlations indicate that TSH associated loci reflect the thyroid function determined by free T3, whereas the FT4 associations represent the thyroid hormone metabolism. Polygenic risk score and Mendelian randomization analyses showed the effects of genetically determined variation in thyroid function on various clinical outcomes, including cardiovascular risk factors and diseases, autoimmune diseases, and cancer. In conclusion, our results improve the understanding of thyroid hormone physiology and highlight the pleiotropic effects of thyroid function on various diseases.

The mediating role of atrial fibrillation in causal associations between risk factors and stroke: a Mendelian randomization study

OBJECTIVES

Atrial fibrillation (AF) contributes to stroke development and progression. We aimed to quantify the mediating role of AF in the causal associations between a wide range of risk factors and stroke via a Mendelian randomization (MR) framework.

METHODS

We assessed the associations of 108 traits with stroke and its subtypes in a 2-sample univariable MR approach, then conducted a bidirectional MR analysis between these 108 traits and AF to evaluate the presence and direction of their causal associations. Finally, to further investigate the extent to which AF mediated the effects of eligible traits on stroke, we applied multivariable and 2-step MR techniques in a mediation analysis where outcomes were restricted to stroke types causally affected by AF (any stroke [AS], any ischemic stroke [AIS], and cardioembolic stroke [CES]).

RESULTS

Among 108 traits, 42 were putatively causal for at least 1 stroke type; of these 42 traits, 20 that had no bidirectional relationship with AF were retained. Finally, 33 associations of 15 eligible traits were examined in the mediation analysis. The mediation analyses for AS, AIS, and CES each included 11 eligible traits. After AF adjustment, the direct effects of all traits on CES were attenuated to null (all p>0.05), while the associations with AS and AIS persisted for most traits (AF-mediated proportion: from 6.6% [95% confidence interval, 2.7 to 0.6] to 52.0% [95% confidence interval, 39.8 to 64.3]).

CONCLUSIONS

The causal associations between all eligible traits and CES were largely mediated through AF, while most traits affected AS and AIS independently of AF.

Age-related variation in thyroid function - a narrative review highlighting important implications for research and clinical practice

BACKGROUND

Thyroid hormones are key determinants of health and well-being. Normal thyroid function is defined according to the standard 95% confidence interval of the disease-free population. Such standard laboratory reference intervals are widely applied in research and clinical practice, irrespective of age. However, thyroid hormones vary with age and current reference intervals may not be appropriate across all age groups. In this review, we summarize the recent literature on age-related variation in thyroid function and discuss important implications of such variation for research and clinical practice.

MAIN TEXT

There is now substantial evidence that normal thyroid status changes with age throughout the course of life. Thyroid stimulating hormone (TSH) concentrations are higher at the extremes of life and show a U-shaped longitudinal trend in iodine sufficient Caucasian populations. Free triiodothyronine (FT3) levels fall with age and appear to play a role in pubertal development, during which it shows a strong relationship with fat mass. Furthermore, the aging process exerts differential effects on the health consequences of thyroid hormone variations. Older individuals with declining thyroid function appear to have survival advantages compared to individuals with normal or high-normal thyroid function. In contrast younger or middle-aged individuals with low-normal thyroid function suffer an increased risk of adverse cardiovascular and metabolic outcomes while those with high-normal function have adverse bone outcomes including osteoporosis and fractures.

CONCLUSION

Thyroid hormone reference intervals have differential effects across age groups. Current reference ranges could potentially lead to inappropriate treatment in older individuals but on the other hand could result in missed opportunities for risk factor modification in the younger and middle-aged groups. Further studies are now needed to determine the validity of age-appropriate reference intervals and to understand the impact of thyroid hormone variations in younger individuals.

Mechanisms and Management of Thyroid Disease and Atrial Fibrillation: Impact of Atrial Electrical Remodeling and Cardiac Fibrosis

Atrial fibrillation (AF) is the most common cardiac arrhythmia associated with increased cardiovascular morbidity and mortality. The pathophysiology of AF is characterized by electrical and structural remodeling occurring in the atrial myocardium. As a source of production of various hormones such as angiotensin-2, calcitonin, and atrial natriuretic peptide, the atria are a target for endocrine regulation. Studies have shown that disorders associated with endocrine dysregulation are potential underlying causes of AF. The thyroid gland is an endocrine organ that secretes three hormones: triiodothyronine (T3), thyroxine (T4) and calcitonin. Thyroid dysregulation affects the cardiovascular system. Although there is a well-established relationship between thyroid disease (especially hyperthyroidism) and AF, the underlying biochemical mechanisms leading to atrial fibrosis and atrial arrhythmias are poorly understood in thyrotoxicosis. Various animal models and cellular studies demonstrated that thyroid hormones are involved in promoting AF substrate. This review explores the recent clinical and experimental evidence of the association between thyroid disease and AF. We highlight the current knowledge on the potential mechanisms underlying the pathophysiological impact of thyroid hormones T3 and T4 dysregulation, in the development of the atrial arrhythmogenic substrate. Finally, we review the available therapeutic strategies to treat AF in the context of thyroid disease.

Long non-coding RNA NBAT1, TUG1, miRNA-335, and miRNA-21 as potential biomarkers for acute ischemic stroke and their possible correlation to thyroid hormones

Objective: RNA-based mechanisms of epigenetic modification related to acute ischemic stroke (AIS) have been widely studied recently. The current work aimed to determine the potential roles of four ncRNAs (TUG1 and its target miR-21, NBAT1, and miR-335) as promising diagnostic biomarkers in AIS as well as their involvement in the disease pathogenesis.

Methods: The levels of the studied lncRNAs and miRNAs were measured in the serum for two different groups, including patients with AIS (60) and healthy controls (60). All individuals were subjected to a full history investigation and clinical examination. Blood samples were tested for FBS, 2HPP, TAG, HDL, LDL, TSH, T3, and T4 levels.

Results: The serum levels of TUG1 were significantly increased in AIS patients compared to control subjects. It is worthwhile to note that serum TUG1 levels were positively correlated with cholesterol, triglycerides, LDL, carotid IMT (Intima-media thickness), and miR-21, while they were negatively correlated with HDL levels. Our study showed that NBAT1 serum expression levels were elevated in AIS patients compared to controls. NBAT1 expression levels were observed to be positively correlated with triglycerides, TUG1, and miR-21. NBAT1 could distinguish between AIS patients and controls with a sensitivity of 100% and specificity of 100% at a cut-off point of 1.45. Regarding miR-335, we found that its expression levels were downregulated in AIS patients compared with healthy controls. It could distinguish between AIS patients and controls with a sensitivity of 73.3% and a specificity of 100% at a cut-off point of 0.796.

Conclusion: Our results revealed that serum TUG1, miR-21, NBAT1, and miR-335 could be promising molecular diagnostic markers for AIS as these biomarkers could discriminate between AIS patients and healthy controls.

Genome-Wide Studies in Ischaemic Stroke: Are Genetics Only Useful for Finding Genes?

Ischaemic stroke is a complex disease with some degree of heritability. This means that heritability factors, such as genetics, could be risk factors for ischaemic stroke. The era of genome-wide studies has revealed some of these heritable risk factors, although the data generated by these studies may also be useful in other disciplines. Analysis of these data can be used to understand the biological mechanisms associated with stroke risk and stroke outcome, to determine the causality between stroke and other diseases without the need for expensive clinical trials, or to find potential drug targets with higher success rates than other strategies. In this review we will discuss several of the most relevant studies regarding the genetics of ischaemic stroke and the potential use of the data generated.

CD11c+ B Cells Participate in the Pathogenesis of Graves’ Disease by Secreting Thyroid Autoantibodies and Cytokines

Graves’ disease (GD) is a common autoimmune disorder with an elevation in pathogenic autoantibodies, specifically anti-thyrotropin receptor antibodies (TRAbs), which are secreted by autoreactive B cells. To date, there has been little research on self-reactive B cells in GD. In the current study, we reported that a unique B-cell subset, CD11c+ B cells, was expanded in the peripheral blood (PB) of GD patients, as detected by flow cytometry. The frequency of CD11c+ B cells was positively correlated with serum TRAb levels. The flow cytometry data showed that CD11c expression was higher in a variety of B-cell subsets and that CD11c+ B cells presented a distinct immunophenotype compared to paired CD11c- B cells. Immunohistochemical and immunofluorescence staining indicated the presence of CD11c+CD19+ B cells in lymphocyte infiltration areas of the GD thyroid. Flow cytometric analysis of PB and fine-needle aspiration (FNA) samples showed that compared to PB CD11c+ B cells, CD11c+ B cells in the thyroid accumulated and further differentiated. We found that CD11c+ B cells from the PB of GD patients were induced to differentiate into autoreactive antibody-secreting cells (ASCs) capable of secreting TRAbs in vitro. Luminex liquid suspension chip detection data showed that CD11c+ B cells also secreted a variety of cytokines, including proinflammatory cytokines, anti-inflammatory cytokines, and chemokines, which might play roles in regulating the local inflammatory response and infiltration of lymphocytes in the thyroid. In addition, we performed a chemotaxis assay in a Transwell chamber to verify that CD11c+ B cells were recruited by thyroid follicular cells (TFCs) via the CXCR3-CXCL10 axis. In conclusion, our study determined that CD11c+ B cells were involved in the pathogenesis of GD in multiple ways and might represent a promising immunotherapeutic target in the future.

Thyroid Function Affects the Risk of Post-stroke Depression in Patients With Acute Lacunar Stroke

Objective

This study aimed to investigate whether thyroid function profiles are associated with post-stroke depression (PSD) and evaluate the mediation effect of cerebral small vessel disease (cSVD) on the association of thyroid function profiles and PSD in patients with acute ischemic lacunar stroke.

Methods

In this study, 372 patients with confirmed acute ischemic lacunar stroke within 3 days of onset were consecutively recruited. Serum levels of thyroid hormones and thyroid antibodies were detected on admission. Lacunar infarcts, white matter lesions, cerebral microbleeds, and enlarged perivascular spaces were rated using validated scales. The severity of depression was scored with the 24-item Hamilton Depression Scale in the hospital after a week of stroke onset. Multivariate regression was utilized to analyze the association of thyroid function profiles and PSD. Mediation analysis was employed to evaluate the effect of cSVD on the association of thyroid function profiles and PSD.

Results

A total of 87 (23.4%) participants were diagnosed with depression after stroke. Serum thyroid-stimulating hormone (TSH) levels were significantly higher in patients with PSD than in those without PSD, while free triiodothyronine (FT3) and free thyroxine (FT4) were not significantly different between the two groups. After adjusting for potential confounders, serum TSH levels were positively associated with the risk of PSD (OR = 1.228; 95% CI: 1.053–1.431, p = 0.009). A similar association was also found between the total cSVD burden score and PSD (OR = 2.137; 95% CI: 1.634–2.793, p < 0.001). Further mediation analysis indicated that 26.37% of the association between TSH and PSD was mediated by cSVD.

Conclusions

Serum TSH levels on admission can probably predict depression after acute ischemic lacunar stroke.

Yearly Incidence of Stroke and Bleeding in Atrial Fibrillation with Concomitant Hyperthyroidism: A National Discharge Database Study

Background: Hyperthyroidism is associated with atrial fibrillation (AF), and the latter is a major risk factor for stroke. Aim: We aimed to investigate the yearly incidence of stroke and bleeding in AF patients with and without concomitant hyperthyroidism from the French National Hospital Discharge Database. Methods: Admissions with AF between January 2010 and December 2019 were retrospectively identified and retrieved from the French national database. Incidence rates of ischaemic stroke and bleeding were compared in AF patients with and without concomitant hyperthyroidism. The associations of risk factors with ischaemic stroke were assessed by Cox regression. Results: Overall 2,421,087 AF patients, among whom 32,400 had concomitant hyperthyroidism were included in the study. During the follow-up (mean: 2.0 years, standard deviation SD: 2.2 years), the yearly incidence of ischaemic stroke was noted to be 2.6 (95% confidence interval CI: 2.5−2.8) in AF patients with concomitant hyperthyroidism, and 2.3 (95%CI: 2.3−2.4) in non-thyroid AF patients. Hyperthyroidism was noted as an independent risk factor for ischaemic stroke (adjusted hazard ratio aHR: 1.133, 95%CI: 1.080−1.189) overall, particularly within the first year of hyperthyroidism diagnosis (aHR 1.203, 95%CI 1.120−1.291), however, the association became non-significant in subsequent years (aHR 1.047, 95%CI 0.980−1.118). Major bleeding incidence was lower in the hyperthyroid AF group in comparison to the non-thyroid AF group (incidence ratio: 5.1 vs. 5.4%/year, p < 0.001). The predictive value of CHA2DS2VASc and HAS-BLED scores for ischaemic stroke and bleeding events, respectively, did not differ significantly between AF patients with or without concomitant hyperthyroidism. Conclusions: Hyperthyroidism seems to be an independent risk factor of ischaemic stroke in AF patients, particularly within the first year of hyperthyroidism diagnosis.

Treating hypothyroidism is not always easy: When to treat subclinical hypothyroidism, TSH goals in the elderly, and alternatives to levothyroxine monotherapy

The majority of patients with hypothyroidism feel better when levothyroxine treatment restores thyroid-stimulating hormone (TSH) concentrations to normal. Increasingly, a significant minority of patients remain symptomatic and are dissatisfied with their treatment. Overzealous treatment of symptomatic patients with subclinical hypothyroidism may contribute to dissatisfaction among hypothyroidism patients, as potential hypothyroid symptoms in patients with minimal hypothyroidism rarely respond to treatment. Thyroid hormone prescriptions have increased by 30% in the United States in the last decade. The diagnosis of subclinical hypothyroidism should be confirmed by repeat thyroid function tests ideally obtained at least 2 months later, as 62% of elevated TSH levels may revert to normal spontaneously. Generally, treatment is not necessary unless the TSH exceeds 7.0-10 mIU/L. In double-blinded randomized controlled trials, treatment does not improve symptoms or cognitive function if the TSH is less than 10 mIU/L. While cardiovascular events may be reduced in patients under age 65 with subclinical hypothyroidism who are treated with levothyroxine, treatment may be harmful in elderly patients with subclinical hypothyroidism. TSH goals are age dependent, with a 97.5 percentile (upper limit of normal) of 3.6 mIU/L for patients under age 40, and 7.5 mIU/L for patients over age 80. In some hypothyroid patients who are dissatisfied with treatment, especially those with a polymorphism in type 2 deiodinase, combined treatment with levothyroxine and liothyronine may be preferred.

Thyroid Function and Risk of Anemia: A Multivariable-Adjusted and Mendelian Randomization Analysis in the UK Biobank

CONTEXT

Thyroid dysfunction is associated with higher anemia prevalence, although causality remains unclear.

OBJECTIVE

This study aimed to investigate the association between thyroid function and anemia.

METHODS

This cross-sectional and Mendelian randomization study included 445 482 European participants from the UK Biobank (mean age 56.77 years (SD 8.0); and 54.2% women). Self-reported clinical diagnosis of hypothyroidism was stated by 21 860 (4.9%); self-reported clinical diagnosis of hyperthyroidism by 3431 (0.8%). Anemia, defined as hemoglobin level of < 13 g/dL in men and < 12 g/dL in women, was present in 18 717 (4.2%) participants.

RESULTS

In cross-sectional logistic regression analyses, self-reported clinical diagnoses of hypo- and hyperthyroidism were associated with higher odds of anemia (OR 1.12; 95% CI, 1.05-1.19 and OR 1.09; 95% CI, 0.91-1.30), although with wide confidence intervals for hyperthyroidism. We did not observe an association of higher or lower genetically influenced thyrotropin (TSH) with anemia (vs middle tertile: OR for lowest tertile 0.98 [95% CI, 0.95-1.02]; highest tertile 1.02 [95% CI, 0.98-1.06]), nor of genetically influenced free thyroxine (fT4) with anemia. Individuals with genetic variants in the DIO3OS gene implicated in intracellular regulation of thyroid hormones had a higher anemia risk (OR 1.05; 95% CI, 1.02-1.10); no association was observed with variants in DIO1 or DIO2 genes.

CONCLUSION

While self-reported clinical diagnosis of hypothyroidism was associated with higher anemia risk, we did not find evidence supporting a causal association with variation of thyroid function within the euthyroid range. However, intracellular regulation of thyroid hormones might play a role in developing anemia.

Association of Thyroid Function with Blood Pressure and Cardiovascular Disease: A Mendelian Randomization

Thyroid function has a widespread effect on the cardiometabolic system. However, the causal association between either subclinical hyper- or hypothyroidism and the thyroid hormones with blood pressure (BP) and cardiovascular diseases (CVD) is not clear. We aim to investigate this in a two-sample Mendelian randomization (MR) study. Single nucleotide polymorphisms (SNPs) associated with thyroid-stimulating hormone (TSH), free tetraiodothyronine (FT4), hyper- and hypothyroidism, and anti-thyroid peroxidase antibodies (TPOAb), from genome-wide association studies (GWAS), were selected as MR instrumental variables. SNPs-outcome (BP, CVD) associations were evaluated in a large-scale cohort, the Malmö Diet and Cancer Study (n = 29,298). Causal estimates were computed by inverse-variance weighted (IVW), weighted median, and MR-Egger approaches. Genetically increased levels of TSH were associated with decreased systolic BP and with a lower risk of atrial fibrillation. Hyperthyroidism and TPOAb were associated with a lower risk of atrial fibrillation. Our data support a causal association between genetically decreased levels of TSH and both atrial fibrillation and systolic BP. The lack of significance after Bonferroni correction and the sensitivity analyses suggesting pleiotropy, should prompt us to be cautious in their interpretation. Nevertheless, these findings offer mechanistic insight into the etiology of CVD. Further work into the genes involved in thyroid functions and their relation to cardiovascular outcomes may highlight pathways for targeted intervention.

Thyroid Function and the Risk of Alzheimer's Disease: A Mendelian Randomization Study

Background: Observational studies suggest an association between thyroid function and risk of dementia, but the causality and direction of these effects are unclear. We aim to test whether genetically predicted variation within the normal range of thyroid function and hypothyroidism is causally associated with the risk of Alzheimer's disease (AD). Methods: Mendelian randomization (MR) analyses using genetic instruments are associated with normal range thyrotropin (TSH) and free thyroxine (fT4) levels. Secondary analyses included investigation of the role of hypothyroidism. Bidirectional MR was conducted to address the presence of a potential reverse causal association. Summary statistics were obtained from the ThyroidOmics Consortium involving up to 119,715 individuals and the latest AD genome-wide association study data including up to 71,880 cases. Results: MR analyses show an association between increased genetically predicted normal range TSH levels and a decreased risk of AD (p = 0.02). One standard deviation increased normal range TSH levels were associated with a decreased risk of AD in individuals younger than 50 years old (p = 0.04). There was no evidence for a causal association between fT4 (p = 0.54) and AD. We did not identify any effect of the genetically predicted full range TSH levels (p = 0.06) or hypothyroidism (p = 0.23) with AD. Bidirectional MR did not show any effect of genetic predisposition to AD on TSH or fT4 levels. Conclusions: This MR study shows that increased levels of genetically predicted TSH within the normal range and in younger individuals are associated with a decreased risk of AD. We observed a marginal association between genetically predicted full range TSH and AD risk. There was no evidence for an effect between genetically predicted fT4 or hypothyroidism on AD. Future studies should clarify the underlying pathophysiological mechanisms.

Clinical-epidemiological and radiological characteristics of stroke patients: A multicentre study

INTRODUCTION AND AIM

Stroke is the leading cause of disability in adults and the second most common cause of death, at a rate of 11.8% worldwide. The purpose of this study was to examine the aetiological, demographic, and clinical characteristics of patients admitted to hospital because of acute strokes.

MATERIALS AND METHODS

This multicentre study retrieved information for all patients admitted to hospital because of an acute cerebrovascular event over a six-month period, and sociodemographic, aetiological, and clinical characteristics were recorded.

RESULTS

A total of 1136 patients, 520 of whom were women (45.7%), with a mean age of 70.3 ± 12.8 years, were included in the study. Of these, 967 were diagnosed with ischaemic stroke (IS) (85.1%), 99 with haemorrhagic stroke (HS) (8.7%), and 70 with transient ischaemic attack (6.1%). The most common risk factor for stroke was hypertension (73%). Carotid disease and hyperlipidaemia rates were higher in patients with HS. Numbers of functionally dependent patients with severe neurological status according to the National Institutes of Health Stroke Scale and modified Rankin scale were significantly higher in the HS group (P < .001). When IS was classified according to the Trial of Org 10172 in Acute Stroke Treatment, small vessel disease emerged as the most common cause (41%). The most common lesion localisations were the parietal lobe (23%) in the IS group and the thalamus (35.3%) in the HS group. Eighty-eight patients (7.7%), 62 (6.4%) in the ischaemic subgroup, and 26 (26.3%) in the haemorrhagic subgroup, died within the first month.

CONCLUSION

Current and accurate evaluations of stroke aetiology are essential for stroke prevention and treatment planning. This study, shows that no change occurred in the aetiology of stroke and epidemiological characteristics and that accurate identification of modifiable stroke risk factors is still a major goal.

Higher thyrotropin leads to unfavorable lipid profile and somewhat higher cardiovascular disease risk: evidence from multi-cohort Mendelian randomization and metabolomic profiling

BACKGROUND

Observational studies suggest interconnections between thyroid status, metabolism, and risk of coronary artery disease (CAD), but causality remains to be proven. The present study aimed to investigate the potential causal relationship between thyroid status and cardiovascular disease and to characterize the metabolomic profile associated with thyroid status.

METHODS

Multi-cohort two-sample Mendelian randomization (MR) was performed utilizing genome-wide significant variants as instruments for standardized thyrotropin (TSH) and free thyroxine (fT4) within the reference range. Associations between TSH and fT4 and metabolic profile were investigated in a two-stage manner: associations between TSH and fT4 and the full panel of 161 metabolomic markers were first assessed hypothesis-free, then directional consistency was assessed through Mendelian randomization, another metabolic profile platform, and in individuals with biochemically defined thyroid dysfunction.

RESULTS

Circulating TSH was associated with 52/161 metabolomic markers, and fT4 levels were associated with 21/161 metabolomic markers among 9432 euthyroid individuals (median age varied from 23.0 to 75.4 years, 54.5% women). Positive associations between circulating TSH levels and concentrations of very low-density lipoprotein subclasses and components, triglycerides, and triglyceride content of lipoproteins were directionally consistent across the multivariable regression, MR, metabolomic platforms, and for individuals with hypo- and hyperthyroidism. Associations with fT4 levels inversely reflected those observed with TSH. Among 91,810 CAD cases and 656,091 controls of European ancestry, per 1-SD increase of genetically determined TSH concentration risk of CAD increased slightly, but not significantly, with an OR of 1.03 (95% CI 0.99-1.07; p value 0.16), whereas higher genetically determined fT4 levels were not associated with CAD risk (OR 1.00 per SD increase of fT4; 95% CI 0.96-1.04; p value 0.59).

CONCLUSIONS

Lower thyroid status leads to an unfavorable lipid profile and a somewhat increased cardiovascular disease risk.

Thyroid Hormone Therapy and Incident Stroke

CONTEXT

Stroke is a leading cause of death and disability and there is a need to identify modifiable risk factors.

OBJECTIVE

We aimed to determine the relationship between thyroid hormone treatment intensity and incidence of atrial fibrillation and stroke.

METHODS

We conducted a retrospective cohort study using data from the Veterans Health Administration between 2004 and 2017, with a median follow-up of 59 months. The study population comprised 733 208 thyroid hormone users aged ≥18 years with at least 2 thyroid stimulating hormone (TSH) measurements between thyroid hormone initiation and incident event (atrial fibrillation or stroke) or study conclusion (406 030 thyroid hormone users with at least 2 free thyroxine [T4] measurements).

RESULTS

Overall, 71 333/643 687 (11.08%) participants developed incident atrial fibrillation and 41 931/663 809 (6.32%) stroke. In multivariable analyses controlling for pertinent factors such as age, sex, and prior history of atrial fibrillation, higher incidence of stroke was associated with low TSH or high free T4 levels (ie, exogenous hyperthyroidism; eg, TSH <0.1 mIU/L; OR 1.33; 95% CI, 1.24-1.43; free T4>1.9 ng/dL, OR 1.17, 95% CI 1.06-1.30) and high TSH or low free T4 levels (ie, exogenous hypothyroidism; eg, TSH >5.5 mIU/L; OR 1.29; 95% CI, 1.26-1.33; free T4 <0.7 ng/dL; OR 1.29; 95% CI, 1.22-1.35) compared with euthyroidism (TSH >0.5-5.5 mIU/L and free T4 0.7-1.9 ng/dL). Risk of developing atrial fibrillation and stroke was cumulative over time for both patients with exogenous hyperthyroidism and hypothyroidism.

CONCLUSION

Both exogenous hyper- and hypothyroidism were associated with increased risk of stroke, highlighting the importance of patient medication safety.

Mendelian Randomization Studies in Stroke: Exploration of Risk Factors and Drug Targets With Human Genetic Data

Elucidating the causes of stroke is key to developing effective preventive strategies. The Mendelian randomization approach leverages genetic variants related to an exposure of interest to investigate the effects of varying that exposure on disease risk. The random allocation of genetic variants at conception reduces confounding from environmental factors and thus strengthens causal inference, analogous to treatment allocation in a randomized controlled trial. With the recent explosion in the availability of human genetic data, Mendelian randomization has proven a valuable tool for studying risk factors for stroke. In this review, we provide an overview of recent developments in the application of Mendelian randomization to unravel the pathophysiology of stroke subtypes and identify therapeutic targets for clinical translation. The approach has offered novel insight into the differential effects of risk factors and antihypertensive, lipid-lowering, and anticoagulant drug classes on risk of stroke subtypes. Analyses have further facilitated the prioritization of novel drug targets, such as for inflammatory pathways underlying large artery atherosclerotic stroke and for the coagulation cascade that contributes to cardioembolic stroke. With continued methodological advances coupled with the rapidly increasing availability of genetic data related to a broad range of stroke phenotypes, the potential for Mendelian randomization in this context is expanding exponentially.

Mendelian Randomization in Stroke: A Powerful Approach to Causal Inference and Drug Target Validation

Stroke is a leading cause of death and disability worldwide. However, our understanding of its underlying biology and the number of available treatment options remain limited. Mendelian randomization (MR) offers a powerful approach to identify novel biological pathways and therapeutic targets for this disease. Around ~100 MR studies have been conducted so far to explore, confirm, and quantify causal relationships between several exposures and risk of stroke. In this review, we summarize the current evidence arising from these studies, including those investigating ischemic stroke, hemorrhagic stroke, or both. We highlight the different types of exposures that are currently under study, ranging from well-known cardiovascular risk factors to less established inflammation-related mechanisms. Finally, we provide an overview of future avenues of research and novel approaches, including drug target validation MR, which is poised to have a substantial impact on drug development and drug repurposing.

Thyroid Function and Mood Disorders: A Mendelian Randomization Study

Background: Observational studies suggest that even minor variations in thyroid function are associated with the risk of mood disorders, including major depressive disorder (MDD) and bipolar disorder (BD). However, it is unknown whether these associations are causal or not. We used a Mendelian randomization (MR) approach to investigate causal effects of minor variations in thyrotropin (TSH) and free thyroxine (fT4) levels on MDD and BD risk. Materials and Methods: We performed two-sample MR analyses using data from the largest publicly available genome-wide association studies on normal-range TSH (n = 54,288) and fT4 (n = 49,269) levels, MDD (170,756 cases, 329,443 controls) and BD (20,352 cases, 31,358 controls). Secondary MR analyses investigated the effects of TSH and fT4 levels on specific MDD and BD subtypes. Reverse MR was also performed to assess the effects of MDD and BD on TSH and fT4 levels. Results: There were no associations between genetically predicted TSH and fT4 levels and MDD risk, nor MDD subtypes and minor depressive symptoms. A one standard deviation increase in fT4 levels was nominally associated with an 11% decrease in the overall BD risk (odds ratio [OR] = 0.89, 95% confidence interval [CI] = 0.80-0.98, p = 0.022) and a 13% decrease in the BD type 1 risk (OR = 0.87, CI = 0.75-1.00, p = 0.047). In the reverse direction, genetic predisposition to MDD and BD was not associated with TSH nor fT4 levels. Conclusions: Variations in normal-range TSH and fT4 levels have no effects on the risk of MDD and its subtypes, and neither on minor depressive symptoms. This indicates that depressive symptoms should not be attributed to minor variations in thyroid function. Borderline associations with BD and BD type 1 risks suggest that further clinical studies should investigate the effect of thyroid hormone treatment in BD.

Genetic Thyrotropin Regulation of Atrial Fibrillation Risk Is Mediated Through an Effect on Height

CONTEXT

A genetic predisposition to lower thyrotropin (TSH) levels is associated with increased atrial fibrillation (AF) risk through undefined mechanisms.

OBJECTIVE

Defining the genetic mediating mechanisms could lead to improved targeted therapies to mitigate AF risk.

METHODS

We used 2-sample mendelian randomization (MR) to test associations between TSH-associated single-nucleotide variations and 16 candidate mediators. We then performed multivariable mendelian randomization (MVMR) to test for a significant attenuation of the genetic association between TSH and AF, after adjusting for each mediator significantly associated with TSH.

RESULTS

Four candidate mediators (free thyroxine, systolic blood pressure, heart rate, and height) were significantly inversely associated with genetically predicted TSH after adjusting for multiple testing. In MVMR analyses, adjusting for height significantly decreased the magnitude of the association between TSH and AF from -0.12 (SE 0.02) occurrences of AF per SD change in height to -0.06 (0.02) (P = .005). Adjusting for the other candidate mediators did not significantly attenuate the association.

CONCLUSION

The genetic association between TSH and increased AF risk is mediated, in part, by taller stature. Thus, some genetic mechanisms underlying TSH variability may contribute to AF risk through mechanisms determining height occurring early in life that differ from those driven by thyroid hormone-level elevations in later life.

Variation in Normal Range Thyroid Function Affects Serum Cholesterol Levels, Blood Pressure, and Type 2 Diabetes Risk: A Mendelian Randomization Study

Background: Observational studies have demonstrated that variation in normal range thyroid function is associated with major cardiovascular risk factors, including dyslipidemia, hypertension, type 2 diabetes (T2D), and obesity. As observational studies are prone to residual confounding, reverse causality, and selection bias, we used a Mendelian randomization (MR) approach to investigate whether these associations are causal or not. Methods: Two-sample MR analysis using data from the largest available genome-wide association studies on normal range thyrotropin (TSH) and free thyroxine (fT4) levels, serum lipid levels, blood pressure measurements, T2D, and obesity traits (body mass index [BMI] and waist/hip ratio). Results: A one standard deviation (SD) increase in genetically predicted TSH levels was associated with a 0.037 SD increase in total cholesterol levels (p = 3.0 × 10-4). After excluding pleiotropic instruments, we also observed significant associations between TSH levels and low-density lipoprotein levels (β = 0.026 SD, p = 1.9 × 10-3), pulse pressure (β = -0.477 mmHg, p = 7.5 × 10-10), and T2D risk (odds ratio = 0.95, p = 2.5 × 10-3). While we found no evidence of causal associations between TSH or fT4 levels and obesity traits, we found that a one SD increase in genetically predicted BMI was associated with a 0.075 SD decrease in fT4 levels (p = 3.6 × 10-4). Conclusions: Variation in normal range thyroid function affects serum cholesterol levels, blood pressure, and T2D risk.

Mendelian Randomization Highlights the Causal Role of Normal Thyroid Function on Blood Lipid Profiles

The association between thyroid function and dyslipidemia has been well documented in observational studies. However, observational studies are prone to confounding, making it difficult to conduct causal inference. We performed a 2-sample bidirectional Mendelian randomization (MR) using summary statistics from large-scale genome-wide association studies of thyroid stimulating hormone (TSH), free T4 (FT4), and blood lipids. We chose the inverse variance-weighted (IVW) method for the main analysis, and consolidated results through various sensitivity analyses involving 6 different MR methods under different model specifications. We further conducted genetic correlation analysis and colocalization analysis to deeply reflect the causality. The IVW method showed per 1 SD increase in normal TSH was significantly associated with a 0.048 SD increase in total cholesterol (TC; P < 0.001) and a 0.032 SD increase in low-density lipoprotein cholesterol (LDL; P = 0.021). A 1 SD increase in normal FT4 was significantly associated with a 0.056 SD decrease in TC (P = 0.014) and a 0.072 SD decrease in LDL (P = 0.009). Neither TSH nor FT4 showed causal associations with high-density lipoprotein cholesterol and triglycerides. No significant causal effect of blood lipids on normal TSH or FT4 can be detected. All results were largely consistent when using several alternative MR methods, and were reconfirmed by both genetic correlation analysis and colocalization analysis. Our study suggested that, even within reference range, higher TSH or lower FT4 are causally associated with increased TC and LDL, whereas no reverse causal association can be found.

Deciphering the Irregular Risk of Stroke Increased by Obesity Classes: A Stratified Mendelian Randomization Study

BACKGROUND

To investigate the relationship between different classes of obesity and stroke, we conducted a stratified Mendelian randomization (MR) study.

METHODS

The body mass index (BMI) data of 263,407 Europeans with three classes of obesity (obesity class I, 30 kg/m² ≤ BMI < 35 kg/m²; obesity class II, 35 kg/m² ≤ BMI < 40 kg/m²; obesity class III, 40 kg/m² ≤ BMI) were extracted from the Genetic Investigation of ANthropometric Traits (GIANT) consortium. Summary-level data of stroke and its subtypes [ischemic stroke (IS) and intracerebral hemorrhage (ICH)] were obtained from the genome-wide association study (GWAS) meta-analysis, which was performed by the MEGASTROKE consortium. MR methods were used to identify the causal relationships.

RESULTS

The MR analysis revealed that both obesity class I [odds ratio (OR) = 1.08, 95% CI: 1.05-1.12, p = 1.0 × 10^-5] and obesity class II (OR = 1.06, 95% CI: 1.03-1.09, p = 1 × 10^-4) were significantly positively related to IS, while obesity class III was not (OR = 1.01, 95% CI: 0.96-1.06, p = 0.65). In contrast to IS, there was no class of obesity associated with ICH risk. Further examination of the relationship between obesity classification and IS subtypes revealed that certain degrees of obesity were related to large artery stroke (LAS) (OR = 1.14, 95% CI: 1.04-1.24, p = 2.8 × 10^-3 for class I; OR = 1.08, 95% CI: 1.01-1.16, p = 0.002 for class II) and cardioembolic stroke (CES) (OR = 1.11, 95% CI: 1.02-1.20, p = 0.02 for class I; OR = 1.08, 95% CI: 1.02-1.15, p = 0.007 for class II).

CONCLUSIONS

A higher risk of IS, but not ICH, could be linked to obesity classes I and II. A strong association between LAS and CES and obesity was observed among all IS subtypes in the obese population.

Does Maternal Normal Range Thyroid Function Play a Role in Offspring Birth Weight? Evidence From a Mendelian Randomization Analysis

BACKGROUND

The association between normal range thyroid function and offspring birth weight has been postulated, but evidence from observational studies is prone to be confounded. We conducted a two-sample Mendelian randomization (MR) study to explore the causal effects of maternal thyroid stimulating hormone (TSH) and free thyroxine (FT4) on birth weight.

METHODS

We utilized public shared summary-level statistics from European-ancestry genome wide association studies. We obtained 40 and 21 single nucleotide polymorphisms as instrumental variables, which were associated with TSH and FT4 levels at genome-wide significance (P < 5 × 10^-8). Partitioned maternal effects on birth weight were retrieved from datasets contributed by the Early Growth Genetics Consortium. Inverse-variance weighted method was employed in the primary MR analysis and multiple sensitivity analyses were implemented.

RESULTS

Genetically determined normal range thyroid function was not causally associated with offspring birth weight. Each one standard deviation (SD) increase in maternal TSH was associated with 0.002 SD higher of birth weight (95% confidence interval [CI], -0.021 to 0.025; P = 0.87). Similarly, change in birth weight was -0.001 SD (95% CI, -0.031 to 0.029; P = 0.94) per one SD higher in maternal FT4. Consistent results were yielded via additional MR methods. Sensitivity analyses demonstrated no presence of horizontal pleiotropy or heterogeneity.

CONCLUSION

This MR study did not identify a causality between normal range thyroid function and offspring birth weight in the Europeans.