Is lower thyroid activity linked to longevity?

NEW YORK (Reuters Health) - A less active thyroid may mean more years added to your life, hints a new Dutch study.

However, the researchers emphasize that the finding, which builds on prior evidence touting the possible link, still does not prove that decreased thyroid function is the fountain of youth -- it may just be related to something else that is.

"In an earlier study, we observed that middle-aged children of long-lived siblings have lower thyroid function compared to controls from the general population," Diana van Heemst of Leiden University Medical Center, in the Netherlands, told Reuters Health in an email.

"In the current study, we sought to assess whether in the generation of the long-lived siblings low thyroid function was related to enhanced survival of the parents of the siblings as well," she said.

The researchers studied 859 siblings from 421 long-lived families. As reported in the Journal of Clinical Endocrinology and Metabolism, their average age of 93 years old far exceeded current U.S. life expectancy of about 78 years.

After rating the longevity of these siblings' parents, the team analyzed the thyroid hormones in the siblings' blood. The two sets of values appeared to be strongly linked, supporting previous findings of heritability in decreased thyroid functioning and its relationship to long life, they say.

This result held up even after accounting for critical illness, which can also affect thyroid activity.

From its location in the neck, the thyroid secretes hormones that affect metabolism. The researchers suggest that the lower activity of thyroid hormones could shift the body's energy expenditure away from growth and proliferation in favor of protective maintenance, keeping the body healthier longer. However, other factors could be associated with both thyroid function and longevity, removing credit from the thyroid.

"These results may come as (a) surprise as low thyroid function is commonly regarded as a risk factor for cardiovascular disease," said van Heemst. "The prevailing recommendation therefore is to treat elderly with low thyroid function with hormone supplementation."

But it is still too early to say whether this practice cheats the elderly of some extra years.

"These data underpin the need for a dedicated clinical trial to test whether treating (mild decreases in thyroid function) with thyroid hormone supplementation is effective in the elderly," van Heemst said.

SOURCE: http://link.reuters.com/weq78n The Journal of Clinical Endocrinology and Metabolism, online August 25, 2010.