New York: Low thyroid activity, one of the most treated conditions in the United States, may actually be a sign of longevity, US researchers have discovered.

While they said it was far too soon for people taking thyroid pills to stop, they will be looking to see if the thyroid may hold the key to a long life, at least for some people.

Dr Martin Surks and colleagues at the Montefiore Medical Center and the Albert Einstein College of Medicine in New York studied hundreds of people who had lived to be 100, and found evidence that people with low thyroid activity were more likely to be in that group.

Dr Surks told a meeting of the Endocrine Society, specialists in human hormones: “We studied a large group of Ashkenazi Jews with exceptional longevity.”

They used a large national survey of health to see what the average hormone levels are for people of various ages.

The thyroid, located in the neck, is a master gland, secreting hormones that affect metabolism. Doctors usually check its activity by an indirect measure — looking at levels of TSH, or thyroid stimulating hormone.

High TSH levels suggest the thyroid is underactive, a condition known as hypothyroidism. Low levels suggest it is overactive, known as hyperthyroidism.

People with low thyroid function may lose hair, gain weight and feel sluggish, while those with overactive thyroids may lose weight, feel their hearts race and have trembling hands Both can be easily treated with a daily pill.

The researchers found 15 to 20 percent of people over the age of 60 had TSH levels that suggest an underactive thyroid gland. He told the meeting he believed that may be normal for older people and may in fact be a sign of longevity.

“We estimate that 70 percent of old people whose TSH was minimally elevated and who were considered to have hypothyroidism were actually in their age-specific limits,” Surks added.

The research looked at 200 Jews who had lived to be 100, and 400 of their children. Two genetic changes were linked with low thyroid function but also with extreme old age.

Metabolic rate affects life span in animals. For instance, elephants have slow metabolic rates, slow heartbeats, and can live for decades, as opposed to mice, which have fast metabolisms and live for just months.

It may be, Surks said, that people with low thyroid function in old age were “elephants” with a slow metabolism who can live longer, as compared to ‘mice” with fast metabolic rates who may have shorter natural life spans.

“If you are an older person with high TSH, this suggests you are on the road to a long life,” Surks said.

What worries him is that millions of people in the United States are being treated for hypothyroidism. “In North America, thyroid hormone is used at the drop of a hat,” he said.

His group is seeking to see if that might interfere with a person’s natural life span.

Surks noted that having a low thyroid function before about age 50 is a separate condition and appropriately treated with hormones.

He also plans studies to see what the biological function of having high TSH levels might mean for cells and ageing.