Subclinical hypothyroidism, which is often the beginning of an underactive thyroid, appears to contribute more substantially toward the risk of heart disease in people up to the age of 65, according to a new study that reviewed the medical literature. The study is being presented Tuesday, June 5, at The Endocrine Society’s 89th Annual Meeting in Toronto.

Subclinical hypothyroidism, which usually causes no symptoms of thyroid disease, is thought to be an early, mild form of hypothyroidism. As in hypothyroidism, the level of thyroid-stimulating hormone is too high in this early form, but unlike hypothyroidism, the level of the thyroid hormone thyroxine is within the normal range for the population, rather than low. Subclinical hypothyroidism affects 8 percent of all women and 3 percent of men, and becomes more prevalent with age, according to the study’s lead author, Dr. Salman Razvi of the United Kingdom’s Royal Victoria Infirmary.

It previously was unclear whether people with this thyroid condition have a higher risk of ischemic heart disease, because results from past studies have been inconsistent, Razvi said. Also called coronary artery disease or coronary heart disease, ischemic heart disease primarily results from narrowing of the arteries supplying the heart. It is the most common type of heart disease, according to the American Heart Association.

To better understand the risk of cardiovascular disease in this thyroid condition, Razvi and co-workers performed a systematic review of 32 studies of subclinical hypothyroidism that assessed ischemic heart disease and its risk factors. The studies were from the United States, Australia, and countries in Europe and Asia.

The occurrence of ischemic heart disease, as well as death due to it, was increased in people with subclinical hypothyroidism but only in studies that included people less than 65 years of age. This result is probably because people older than 65 increasingly have other risk factors for cardiovascular disease, so the risk due to thyroid abnormality becomes diluted, Razvi speculated.

The researchers also found that treatment of subclinical hypothyroidism reduced LDL (“bad”) cholesterol and body mass index, a measure of obesity.

“Our analyses of large data sets suggest that there are age-related differences in the risk of cardiovascular disease attributable to subclinical hypothyroidism,” Razvi said. “This has potentially important implications for therapy.”

Patients with subclinical hypothyroidism may receive treatment with thyroid hormone medications if their doctors advise it, according to Razvi. 

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