News Article
Subclinical Thyroid Dysfunction Increases Cardiovascular, All-Cause Mortality
CHICAGO (EGMN) - Subclinical hyperthyroidism significantly increases the risks of both cardiovascular and all-cause mortality, according to a prospective Brazilian cohort study.
Subclinical hypothyroidism was similarly an independent risk factor for all-cause mortality in the study. But its association with increased cardiovascular mortality fell short of statistical significance, reported Dr. Jose Augusto Sgarbi of the University of Marilia (Brazil).
He added that there have been conflicting results in the few well-designed epidemiologic studies that have previously addressed the question of a possible increase in cardiovascular mortality in patients with subclinical thyroid dysfunction. As a result, there has been no consensus regarding screening and treatment of these abnormalities.
Thus, the Brazilian findings have important clinical implications: "These data highlight the importance of a more aggressive strategy to identify and treat individuals with subclinical thyroid dysfunction," the endocrinologist said at the annual meeting of the American Thyroid Association.
He reported on 1,100 adult participants in the longitudinal Japanese-Brazilian Thyroid Study. None were on thyroid medications at baseline.
At baseline, subclinical hypothyroidism - defined as an abnormally low serum TSH level in the presence of normal serum free T4 and total or free T3 - was identified in 6.2% of the study population. Subclinical hyperthyroidism was detected in 8.9%. Overt hyperthyroidism was found in 1.8%, while 0.8% had overt hypothyroidism. The remaining subjects were euthyroid.
During a mean 7 years of follow-up, 76 participants died. The overall mortality rate was 5.1% in euthyroid individuals and significantly higher at 12.1% in those with subclinical hypothyroidism and 21.7% in participants with subclinical hyperthyroidism.
All-cause and cardiovascular mortality were higher in subjects with more pronounced subclinical thyroid dysfunction than in those with milder abnormalities. For example, the cardiovascular mortality rate was 2.8% in euthyroid subjects, 8.6% in those with relatively mild subclinical hyperthyroidism as defined by a TSH of 0.1-0.44 mU/L, and more than 27% in participants with a TSH below 0.1 mU/L. But even in individuals with only mildly increased or decreased TSH the increase in mortality was statistically significant.
Subjects with subclinical thyroid dysfunction tended to be older than euthyroid individuals. However, the populations had similar baseline rates of dyslipidemia, diabetes, hypertension, metabolic syndrome, smoking, and macrovascular disease.
In a multivariate Cox regression analysis that controlled for age and other potential confounders, subclinical hyperthyroidism was independently associated with a 3.4-fold increased risk of all-cause mortality compared with euthyroid status. Subclinical hypothyroidism conferred a 2.4-fold increased risk.
Individuals with subclinical hyperthyroidism were at 5.5-fold increased risk for cardiovascular mortality. The 1.9-fold increase in subclinically hypothyroid subjects was not statistically significant. The elevated all-cause mortality in this group was attributed to a combination of cardiovascular, cancer, and infectious disease deaths.
In the subclinically hyperthyroid group, all-cause and cardiovascular mortality rates were significantly increased among both men and women. In those with subclinical hypothyroidism, however, the elevated mortality risk was present only in men.
The increased mortality associated with subclinical thyroid dysfunction was confined to patients aged older than 60 years.
Dr. Orlo H. Clark, an audience member, wondered if the Brazilian findings could be extrapolated to patients who have had surgery for thyroid cancer.
"Are we hurting patients when we purposely suppress their TSH once they've had thyroid cancer surgery? Do you think that has an adverse effect on their survival?" asked Dr. Clark, professor and chair of the department of surgery at the University of California, San Francisco, Medical Center at Mount Zion.
Dr. Sgarbi replied that he believes that's a reasonable implication, although the study didn't address that issue.
The Japanese-Brazilian Thyroid Study was funded by a federal agency.
All articles have been reviewed by members of our Editorial Board or independent referees.
You need Adobe Acrobat Reader installed on your computer to read the PDF version of these articles. If you don't have Acrobat Reader, click here to download it free.
