The data suggest that silent or "subclinical" thyroid dysfunction "might represent a potentially modifiable -- albeit modest -- risk factor for coronary heart disease and mortality," said Dr. Nicholas Rodondi, from the University of Lausanne in Switzerland.

Their findings are based on pooled data from 12 studies identified through a search of MEDLINE (1950 to 2008). Ten of the studies involved population-based groups that included 14,449 subjects.

The likelihood of coronary heart disease, heart-related death, and death from any cause was higher by 20 percent, 18 percent, and 12 percent, respectively, in subjects with an underactive thyroid without symptoms -- also referred to as subclinical hypothyroidism.

People with an overactive thyroid but without symptoms (i.e., silent hyperthyroidism) had a 21 percent, 19 percent, and 12 percent greater odds, respectively, of heart disease, heart-related death, and death from any cause.

Rodondi said studies are needed to determine the impact of treating these two conditions on heart disease risk.

All of the population-based studies examined the impact of subclinical hypothyroidism on heart disease and mortality, whereas only five looked at the effect of subclinical hyperthyroidism.