Low thyroid activity, one of the most treated conditions in the United States, may be a sign of longevity, researchers reported on Friday.
Although they said it is far too soon for people taking thyroid pills to stop, they will be checking to see whether the thyroid may hold the key to a long life, at least for some people.
The thyroid, which is in the neck, is a kind of master gland, secreting hormones that affect metabolism. Doctors usually check its activity with an indirect measure such as 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 treated easily with a daily pill.
Surks and colleagues found that 15 percent to 20 percent of people over 60 had TSH levels that suggest an underactive thyroid gland. He said he believes that may be normal for older people and could 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 said in a telephone interview.
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 could be that people with low thyroid function in old age were "elephants" with a slow metabolism and can live longer compared with "mice" with fast metabolic rates and may have shorter natural life spans, Surks said.
"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.
His group is researching whether 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.
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