The relationship between hypothyroidism and atrial fibrillation (AFib) is complex and not fully understood. Some studies suggest an association, while others propose a potential protective effect.

Thyroid hormones can have a significant impact on the function of your heart and vascular system.

Thyroid disease, particularly hyperthyroidism, is a known risk factor for atrial fibrillation (AFib), an irregular and often rapid heart rate that can increase the risk of stroke and heart failure.

However, it’s less clear how hypothyroidism — a thyroid condition involving insufficient hormone production — affects AFib.

Although hypothyroidism can trigger other cardiovascular risk factors associated with AFib, a direct connection between hypothyroidism and AFib remains less clear.

AFib can be associated with hypothyroidism and hyperthyroidism, although a relationship with hyperthyroidism is more commonly recognized.

A few studies suggest that hypothyroidism is notably prevalent in AFib, potentially surpassing hyperthyroidism.

For example, in a 2022 study of 2,000 people with AFib, researchers found that 10.5% had thyroid disease. Of those, 90% had hypothyroidism, 6.1% had hyperthyroidism, and 3.3% had typical thyroid function.

However, in the broader context, hypothyroidism is more prevalent than hyperthyroidism. For example, in women, these conditions occur at rates of 5.9% and 2%, respectively.

Thyroid disease is a known risk factor for AFib.

In particular, the link between AFib and hyperthyroidism is well established. This makes sense because hyperthyroidism itself can increase heart rate and cause atypical heart rhythms and palpitations.

However, the association between AFib and hypothyroidism has been less studied, and available research results are mixed. Some studies even suggest that hypothyroidism may have a protective effect against AFib.

For instance, a large registry study published in 2012 from Denmark reported that hypothyroidism was associated with even lower rates of AFib than occurred in people with typical thyroid function.

A 2017 study found no significant association between mild hypothyroidism and AFib risk. However, hyperthyroidism — higher thyroid hormone levels within the typical range — appeared to correlate with an increased AFib risk.

In contrast, a 2013 study in rats concluded that both hypothyroidism and hyperthyroidism increased the rats’ vulnerability to AFib. But findings in animal studies don’t always translate to humans.

Further research is needed to better clarify this association.

What TSH level can cause AFib?

The relationship between thyroid-stimulating hormone (TSH) levels and the risk of AFib is complex. It is not as straightforward as a specific TSH level causing AFib.

In a 2017 study with 30,085 participants, baseline TSH levels did not significantly affect AFib risk in populations with typical thyroid function or subclinical hypothyroidism. Instead, higher free thyroxine levels within the expected thyroid range showed a link to increased AFib risk.

A 2014 study also indicated a trend of lower AFib risk in people with higher TSH levels, which occur in hypothyroidism. But this trend wasn’t statistically significant, likely due to the limited number of AFib events in this group.

A 2008 study found that lower TSH levels within the typical range (0.4–4.0 milliunits per liter) and higher levels of free thyroxine in the typical range (0.86–1.94 nanograms per deciliter) were linked to a greater risk of AFib in older adults.

However, this study does not state a specific TSH level that directly causes AFib. It indicates that even levels within the typical range might affect AFib risk.

Hypothyroidism is associated with several cardiovascular risk factors that can indirectly contribute to an increased risk of AFib:

In addition, research suggests that hypothyroidism can influence heart rhythm as well as how the heart pumps and how the blood vessels respond.

These factors may collectively contribute to a potential indirect link between hypothyroidism and AFib.

AFib treatment in people with hypothyroidism typically involves addressing both conditions.

AFib can be managed in two main ways: rate control, which aims to keep the heart rate within a certain range, and rhythm control, which aims to restore and maintain a standard heart rhythm.

The rate- and rhythm-control strategies refer to the types of medications prescribed, with rate-control focusing on managing the pace of the heart and rhythm-control focusing on the consistency of the heartbeat.

It’s important to note that levothyroxine, the medication commonly used for hypothyroidism, can occasionally worsen AFib. While levothyroxine effectively corrects thyroid hormone levels in people with hypothyroidism, caution is advised to avoid excessive dosing to prevent AFib from worsening.

For instance, one 2021 study found that higher doses of levothyroxine (above 0.075 milligrams per day) were linked to a greater risk of AFib in older adults with thyroid disease.

Thyroid disease is a known risk factor for AFib, but the direct link between hypothyroidism and AFib is less clear. While hypothyroidism can affect the heart and thus have an indirect effect on AFib, some studies suggest that hypothyroidism might be associated with a lower risk of AFib or even have a protective effect.

Overall, the connection between hypothyroidism and AFib is complex and may be influenced by various factors, including the severity of thyroid dysfunction, your overall health, and other cardiovascular risk factors.