MONDAY, April 23 (HealthDay News) -- New evidence suggests that a type of overactive thyroid condition appears to boost the risk of heart problems, especially atrial fibrillation (a form of irregular heartbeat) and premature death.

Patients sometimes are reluctant to do anything about the condition, known as subclinical hyperthyroidism, because it often doesn't cause any symptoms. The findings show, however, that "physicians and patients should take it seriously and consider the appropriate way to treat it to prevent increases in heart disease, bone problems and death," said Dr. Kenneth Burman, chief of the endocrine section at Washington Hospital Center, in Washington, D.C.

Patients with subclinical hyperthyroidism have too much of the hormone created by the thyroid gland, which helps control people's metabolism. An estimated 10 percent of the population has the condition, which is considered to be less serious than overt hyperthyroidism.

Researchers have wondered for years whether subclinical hyperthyroidism puts people at risk of a variety of health problems. Previous research has suggested it does, and a new study takes a closer look and finds more reasons to suspect the condition is dangerous.

The report authors examined the results of 10 studies, which included nearly 53,000 participants. After adjusting their statistics so they wouldn't be skewed by high or low numbers of participants of certain ages or genders, the researchers found that those with subclinical hyperthyroidism were 24 percent more likely to die during the study periods, 29 percent more likely to die of heart-related problems and 68 percent more likely to have atrial fibrillation.

Burman, who wrote a commentary accompanying the study, said the risk of early death and heart problems were still low even with the increased risk. The risk of death during the study period, for example, rose overall from 16 percent in those with normal thyroid levels to 18 percent in those with subclinical hyperthyroidism. But the heightened risk of atrial fibrillation was a significant jump, he said. Atrial fibrillation causes the heart to fail to beat properly, putting patients at higher risk of stroke.

What to do? Physicians often turn to medication first, then surgery or treatment with radioactive iodine, Burman said. But medication raises questions, he said: "Do you keep them on medication indefinitely when they feel fine and the medications have side effects?"

Study co-author Dr. Nicolas Rodondi, head of ambulatory care at the University of Bern, in Switzerland, said treatment should be considered if patients are in certain risk groups and only if their thyroid levels remain abnormal after they're rechecked in three to six months.

The next step in research is to confirm the analysis findings and explore how treatment may help patients lower their risks of problems, he said.

The study appears online April 23 in the journal Archives of Internal Medicine. A second study, also published in the journal, examined whether the drug levothyroxine sodium -- a man-made form of the thyroid hormone -- would help reduce the risk of cardiovascular problems in patients with subclinical hypothyroidism.

The study of about 4,800 patients, led by researchers at Newcastle University in England, found that the drug (brand names include Synthroid), appeared to reduce the risk of heart problems in relatively younger patients (aged 40 to 70) but not in older patients (over 70).

In younger patients, about 4 percent of those treated with the drug had heart disease, compared with nearly 7 percent of those who weren't treated with it. After adjusting their statistics so they wouldn't be skewed by various factors, the researchers found that those who took the drug had a 39 percent lower risk of heart disease.

The drug can, however, cause a variety of side effects. Researchers could not definitively explain why older patients didn't receive the same health benefit.

A co-author for this study has received a speaking fee from drug manufacturer Merck Serono.

More information

For details about thyroid diseases, try the U.S. National Library of Medicine.