Article

AIM:  the aim of this study is to evaluate the association between SCH and the risk of cardiovascular disease (CVD) and cardiovascular mortality. 

Sample size: N= 49,201

• 17 cohort studies were included in the meta- analysis.
• The mean length of the follow-up ranged from 2 to 20 years.

Inclusion Criteria: 

• Cohort studies published between 2001-2011 
• Studies published in the western countries 
• TSH cut-off value of 0.25–0.5 mU/l with normal free thyroxine (FT4) level
• CHD was defined as acute myocardial infarction, angina pectoris, and other ischemic heart disease. 

Exclusion Criteria 

• Exposures not relevant
• Not cohort studies 
• Case-reports 
• Review articles

Important finding 

• Based on this meta-analysis subclinical Hyperthyroidism(SCH) can significantly increase the risk of cardiovascular disease (CVD) in the general population but it can increase cardiovascular mortality in patients with other comorbidities. 

• In the general population, an individual with SCH has 31%  increased risk of CVD compare to someone who doesn’t have SCH
• all studies made adjustments for conventional cardiovascular risk factors, including age, BMI, blood pressure, diabetes, cholesterol, and smoking, suggesting that SCH is probably an independent risk factor for CVD.
• This study also showed SCH has been found to be associated with some cardiovascular risk factors such as: atrial fibrillation, hypertension, increased factor X activity, and increased levels of plasma von Willebrand factor , fibrinogen, and D-dimer.. These  increases in cardiovascular risk factors can  predisposed individuals to increased CVD risk.
• This study does not suggest a clinical intervention for SCH because the small size was small and the follow up was for a relatively short period of time to suggest intervention to reduce cardiovascular events and mortality. The author suggests well-designed clinical trials are warranted to address  questions regarding intervention.

Strength of the study 

• This study  included all original cohort studies compared to previous meta-analysis that used cross sectional studies. The cohort studies are more powerful to detect a causal relationship than a cross-sectional study. 
• This meta-analysis  adjusted for important confounders factors such as  age, smoking, diabetes
• Based on the Egger’s regression test this study has little evidence of publication bias (PZ0.31–0.84 for all associations).

Drawbacks of the Study 

• Various definitions of SCH were used between the 17 cohort studies. 
• There are heterogeneity among studies for the association between SCH and the risk of mortality