For such a little gland, the butterfly-shaped thyroid has some mighty big effects on every aspect of your metabolism. So when thyroid function is off-kilter, the consequences can be major. In fact, the consequences can affect virtually your whole body—including that most vital of vital organs, your heart. And you wouldn’t necessarily know it.

The connection: When the thyroid gland is overactive, it can trigger a heart-rhythm disorder known as atrial fibrillation, or A-fib, in which the heart beats chaotically. And A-fib increases the risk for stroke and heart failure.

Here’s the problem: A worrisome new study shows that even when thyroid activity is elevated by only a barely detectable amount, you still are at increased risk for A-fib and its potentially deadly consequences.

Here’s what you need to know to determine whether your thyroid is pushing your heart toward the danger zone…

A REALLY BIG STUDY ON A LITTLE GLAND

In a new Danish study, researchers reviewed the records of nearly 600,000 adults. All had had their thyroid function tested for the first time and had no previous diagnosis of thyroid disease or A-fib. Testing included measuring levels of a pituitary gland hormone called thyroid stimulating hormone (TSH), whose job it is to stimulate the thyroid gland to produce thyroid hormones. Abnormally low TSH levels indicate that the thyroid gland is overactive.

Researchers then followed the participants for an average of 5.5 years to see who ended up with A-fib. Results: Compared with people whose thyroid function was normal, those with hyperthyroidism were 42% more likely to develop A-fib. That’s not surprising.

What was startling was that people with subclinical hyperthyroidism—meaning that their TSH levels were low but their thyroid hormone levels were OK and thus they did not warrant a diagnosis of hyperthyroidism—had a 30% increased risk for A-fib. And here’s the real shocker: Participants whose TSH levels were on the low end yet still within the normal range—meaning that they did not have even subclinical hyperthyroidism, just slightly elevated thyroid activity—had a 12% increased risk for A-fib.

HELP FOR YOUR HEART

I contacted the lead author of the study, Christian Selmer, MD, to discuss self-defense strategies. He told me that one key is to be on the lookout for even vague symptoms that might suggest your thyroid is becoming overactive.

Symptoms to watch for:

Sudden unexplained weight loss
Increased appetite
Insomnia
Fatigue
Racing heartbeat
Hot flushes, sweating or heat intolerance
Trembling
More frequent bowel movements or diarrhea
Brittle or thinning hair
Swelling at the base of the neck
Anxiety
Nervousness
Irritability.

Should you notice any such symptoms, even if they are minor, report them to your doctor without delay. Also ask for a blood test to check your TSH level.

However: Being alert to possible symptoms of an overactive thyroid is all well and good—in fact, it’s essential, as we’ve said. But a person with subclinical hyperthyroidism or low TSH would not be likely to have noticeable symptoms—yet he or she would still be at increased risk for A-fib, as the study showed. So how can you protect yourself in that case? A reasonable approach is to tell your doctor that you want a TSH measurement to be part of any routine blood test.

If your TSH test reveals any irregularity—even a slight one that suggests only mildly elevated thyroid activity—it is important to have an electrocardiogram (ECG) to check for A-fib. In fact, Dr. Selmer said, a person who is diagnosed with any degree of overactive thyroid should have an ECG as a part of every routine checkup going forward.

Depending on the severity of your hyperthyroidism, your doctor may prescribe medications to reduce thyroid activity, along with supplements and dietary changes to help prevent complications. Good control of your hyperthyroidism should reduce your A-fib risk.

What if you do develop A-fib? The condition can be treated with medications and/or procedures that reset the heart’s rhythm and control heart rate…plus lifestyle changes to support overall heart health. Appropriate A-fib treatment will go along way toward protecting you from stroke and other serious cardiovascular threats.