NEW YORK - A new report suggests higher-than-normal thyroid activity is linked to a greater risk of heart disease, including irregular heartbeats.
People with so-called subclinical hyperthyroidism have a slightly-overactive thyroid - a gland at the front of the neck responsible for regulating metabolism by releasing certain hormones.
Those people often don't have the typical signs of full-on hyperthyroidism, such as increased appetite, restlessness and fatigue - so the condition may frequently go undiagnosed.
Because many patients just outside the normal range are older and their thyroid function and hormones can naturally fluctuate, doctors have wondered whether or not it's helpful to treat them with thyroid drugs.
"There's been a discussion by many of us about what we should do with subclinical hyperthyroidism, because the risks were unclear," said Dr. Nicolas Rodondi from the University of Bern, Switzerland, who worked on the report.
Slightly-overactive or underactive thyroids are much more common than overt thyroid disease, researchers said.
Full hypo- and hyperthyroidism have been clearly tied to the body's inability to regulate its normal functions, and can be treated with medication or surgery.
Rodondi's study, published in the Archives of Internal Medicine last Monday, suggests subclinical hyperthyroidism may in fact carry extra risks of its own.
In an analysis of previous trials including over 50,000 older adults, people whose hormone tests showed slight hyperthyroidism were 29 per cent more likely to die of heart disease over the next nine years than those with normal thyroid function.
Subclinical hyperthyroidism was also tied to a higher risk of irregular heartbeat - but not to stroke or cancer, Rodondi and his colleagues reported.
Rodondi's group defined "normal" thyroid function as blood levels of thyrotropin, also known as thyroid stimulating hormone (TSH), between 0.45 and 4.49 milli-international units per liter.
The normal range for TSH itself remains a topic of debate, however, with some experts favoring a narrower definition of 0.3 to 3.0 mIU/L. That would potentially give some 20 per cent of Americans "abnormal" TSH levels.
Too much or too little 'a problem'
TOO MUCH OR TOO LITTLE "A PROBLEM"
It's possible that any variation outside the normal range of thyroid function, whether high or low, could increase heart risks, according to Dr. Salman Razvi, of Gateshead Health National Health Service Foundation Trust and Newcastle University, UK.
Too little thyroid activity is tied to a slower metabolism, which can come with weight gain and weight-associated cardiovascular problems.
"The heart relies on thyroid hormone to function normally," Razvi told Reuters Health. "It's like diabetes: if you have too much (blood) sugar you have a problem, and if you have too little sugar, you have a problem."
In a separate study published in the same journal, a group of researchers led by Razvi found that treating subclinical hypothyroidism -- thyroid glands that are slightly less active than normal -- with thyroid hormone was linked to fewer long-term heart problems.
Those findings came from a database of general practitioners who saw close to 5,000 people with underactive thyroids starting in 2001. About half of patients were prescribed the thyroid drug levothyroxine.
Among people aged 40 to 70, just over four per cent of levothyroxine-treated patients developed heart disease during the next eight years, compared with close to seven per cent who weren't treated for low thyroid function.
There was no difference in the risk of heart disease for patients older than 70 who did or didn't take levothyroxine, however, Razvi and his colleagues found.
Adding to the list of potential health concerns with subclinical hypothyroidism, a third study recently published in Obstetrics & Gynecology found that women with slightly-underactive thyroids were more likely to develop diabetes during pregnancy than those with normal thyroid function.
WHO TO TEST?
Rodondi pointed out that the new research can't prove slightly-off thyroid function leads to heart disease or other health problems.
To make that claim, he said, would require a "gold standard" controlled trial, in which people just outside the normal thyroid range are randomly assigned to be treated or not and followed for the incidence of new disease diagnoses.
Dr. H. Gilbert Welch, who studies disease detection and overdiagnosis at the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, New Hampshire, said he thinks it's time for that type of trial. But he called for a study that starts even earlier, when symptom-free people are split into groups of those who do or don't undergo screening for thyroid problems.
"I think that's the real question: should doctors be looking for thyroid abnormalities in patients who feel fine?" Welch, who wasn't involved in the new research, told Reuters Health.
If screening does prove to be helpful, Welch said, the questions become: what level of thyroid function counts as "abnormal"? And, what's the best treatment strategy?
"If you've got full-blown hypo- or hyperthyroidism, that does require treatment," Razvi said. "It's this mild, subclinical form which is the controversial one."