It’s fairly common for people to have an under-functioning thyroid with no obvious ill-effects - a condition called subclinical hypothyroidism - but now it seems this may raise their risk of heart disease, Japanese investigators have found.

Dr. Misa Imaizumi of the Radiation Effects Research Foundation in Nagasaki, and colleagues, looked for a possible relationship between subclinical hypothyroidism and artery disease in 2550 subjects who participated in thyroid disease screening between 1984 and 1987.

Heart disease developed 2.6-times more frequently among participants with subclinical hypothyroidism than those with normal thyroid function, the team reports in the Journal of Clinical Endocrinology and Metabolism.

Men were more likely to be affected than women. The prevalence of heart disease was increased to a greater extent in men (3.7-fold) than in women (1.6-fold) with subclinical hypothyroidism.

During the 10 years following thyroid testing, survival was lower in men (but not women) with subclinical hypothyroidism, the researchers report.

Hypothyroidism is marked by elevated levels of thyroid stimulating hormone, necessary for the under-functioning gland to maintain normal output of thyroid hormones. However, the thyroid may not be the only organ affected by thyroid stimulating hormone, which may explain the current findings.

Receptors for thyroid stimulating hormone have been “recently reported to be expressed on coronary arteries” as well as on fat cells that secrete various factors “that affect the development of atherosclerotic diseases,” Imaizumi explained to Reuters Health.

In this way, elevated thyroid stimulating hormone levels may “induce heart disease in subclinical hypothyroidism,” Imaizumi said, adding that people with subclinical hypothyroidism should therefore be watched carefully for signs of heart trouble.

SOURCE: Journal of Clinical Endocrinology and Metabolism, July 2004.