1. Thyroid stimulating hormone (TSH) levels within the normal range were not associated with increased risk of coronary heart disease (CHD) events or mortality.
2. Additionally, there was no significant difference in risk of CHD mortality or events between patients in the lowest TSH level group as compared to the patients in the highest TSH level group.
Evidence Rating Level: 2 (Good)
Study Rundown: It has been thought that hypothyroidism is associated with coronary heart disease (CHD). Additionally, among individuals within the normal thyroid stimulating hormone (TSH) range, those at the higher end may have early hypothyroidism. The results from previous large observational studies looking at TSH levels and its association with CHD have been varied. This study aims to use individual patient data from 14 observational cohorts to determine if levels of TSH in the upper range of normal are associated with increased risk of CHD.
Thyrotropin levels within the reference range of 0.45 to 4.49 mIU/L were not significantly associated with an increased risk of CHD events or mortality. These results remained robust through all subgroup analyses and adjustments. Strengths of this study include its large sample size and use of individual patient data, which allowed for better classification of exposures and covariates. Limitations for this study include only having one baseline TSH level for the exposure. It would be of interest to further study patients who have TSH levels that remain in the upper range of normal to determine CHD risk.
Relevant Reading: Thyroid hormone and heart failure
In-Depth [meta-analysis]: This study used individual patient data from 14 cohort studies that examined the relationship between CHD risk and varying thyroid function. The cohorts included occurred between July 1972 and April 2002. Authors of the cohorts were contacted to obtain individual patient data (i.e., cardiovascular risk factors, thyroid function measurements, comorbidities and outcomes) and resulted in a total participant count of 55 412 patients with 643 183 person-years of follow-up. The exposure of interest was thyroid function as measured by serum TSH. Outcomes analyzed were evidence of CHD mortality and CHD events. Hazard ratios were adjusted for age, sex and smoking status.
Among 55 412 individuals, 1813 (3.3%) died from complications of CHD during follow-up. Both coronary heart disease mortality (adjusted HR 0.97; 95%CI 0.90-1.04) and first-time CHD event (aHR 1.00; 95%CI 0.97-1.03) were not significantly associated with TSH levels. When analyzed by categories of TSH levels, hazard ratios for the lowest TSH level compared to the highest TSH level for both CHD mortality and CHD events remained non-significant.
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