When Metabolism Slows, Disease May Be a Heartbeat Away

Some people might interpret lethargy, fatigue, dry skin, weight gain, and a constant chill as a sign that their bodies are aching to hibernate in the face of the coming winter. But this suite of symptoms can also signal an underactive thyroid gland, one that produces too little of certain hormones that regulate metabolism.

Full-blown hypothyroidism affects only 2 to 3percent of Americans, most of whom take daily pills containing synthetic thyroid hormones. But a surprising number of people—somewhere between 5 and 15 percent of Americans—have a milder form of thyroid dysfunction known as subclinical hypothyroidism. Most of these people are not treated, and they tend to have few or no symptoms. But new research suggests that people with subclinical hypothyroidism may be at risk for heart failure and other health problems.

A large study presented this month at the American Thyroid Association's annual meeting in New York City showed that senior citizens with mild hypothyroidism are twice as likely to develop congestive heart failure as people with normal thyroid function. The former group had a 4.5 percent annual risk of developing heart failure, whereas the latter group had a 2.2 percent risk. Previous studies had established tenuous links between mild hypothyroidism and a host of health problems, including other cardiovascular diseases and certain cognitive problems. "This is a potentially reversible cause of heart failure," says Douglas Bauer, a professor of medicine and epidemiology at the University of California-San Francisco, who led the new study.

A second study presented at the meeting found that mothers who'd had mildly underactive thyroids during pregnancy were more likely to have babies with vision defects. "Thyroid hormone is critical for development" of the vision-related areas of the brain, says study leader Joanne Rovet, a professor of pediatrics and psychology at the University of Toronto. Other research has shown that pregnant women with mild hypothyroidism face elevated odds of miscarrying or delivering preterm.

The new research suggests that more people—in particular, more women who might become pregnant—should be getting tested for thyroid problems, some experts say. "I think the evidence is moving in favor of screening all women of reproductive age," says Rovet. She points out that proper levels of thyroid hormone are especially important during the first few months of pregnancy (before many women know they're pregnant).

However, other experts say the evidence is still too weak to recommend screening for or treatment of subclinical hypothyroidism. Most medical organizations, including the American College of Family Physicians, recommend screening only for older women and people with other recognized risk factors, such as an autoimmune disease or a family history of thyroid disease.

Experts recommend that people who think they may have hypothyroidism or who are experiencing possible thyroid-related symptoms should get tested. "It's a very simple test to get," says Gregory Brent, a UCLA professor of physiology and spokesperson for the American Thyroid Association. It involves drawing a blood sample and testing its hormone levels.

For those found to have subclinical hypothyroidism, a doctor might prescribe a daily pill containing synthetic thyroid hormone. That's the standard treatment for pronounced hypothyroidism. But "we don't know yet if there's a benefit to treating people with mild thyroid conditions," says Bauer. "There needs to be a clinical trial to look at this."