Yes, your thyroid affects your heart, and it does so in two different directions. An underactive thyroid raises LDL cholesterol. An overactive thyroid raises the risk of atrial fibrillation, an irregular heart rhythm.
Both are found with a simple blood test. That is why an unexplained rise in cholesterol, or a new irregular heartbeat, often prompts a thyroid check before anything else is decided.
Key takeaways
- An underactive thyroid slows the liver's clearance of LDL, so cholesterol and ApoB go up.
- Mildly abnormal thyroid tests carry heart risk mainly at the more abnormal end of the range.
- Even a slightly overactive thyroid is linked with more atrial fibrillation.
- Knowing your thyroid status helps make sense of a cholesterol result before treatment is discussed.
Two directions, two different heart effects
This article covers only the heart side of thyroid disease. For symptoms, causes and how thyroid conditions are diagnosed, see the clinic's thyroid page.
The main blood test is TSH, the hormone your brain uses to drive the thyroid. A high TSH usually means the thyroid is underactive. A low TSH usually means it is overactive.
| Thyroid state | Typical TSH | Main heart-related effect |
|---|---|---|
| Underactive (hypothyroidism) | High | Higher LDL cholesterol and ApoB |
| Mildly underactive (subclinical) | High, with normal thyroid hormone | More coronary events at the highest TSH levels |
| Mildly overactive (subclinical) | Low, with normal thyroid hormone | More atrial fibrillation |
An underactive thyroid raises LDL
Thyroid hormone helps set the number of LDL receptors on liver cells. These receptors pull LDL particles out of the blood.
A review in the journal Thyroid describes what happens when the hormone runs low. Overt hypothyroidism brings high cholesterol and a marked increase in LDL and apolipoprotein B, because the liver has fewer LDL receptors and clears LDL more slowly.1 The review notes that it is often accompanied by raised diastolic blood pressure as well.
The effect is reversible. According to the same review, thyroid hormone replacement usually leads to a considerable improvement in the lipid profile.1 If you want the background on why particle count matters, our ApoB test guide explains it.
Mildly underactive: does it matter for the heart?
Subclinical hypothyroidism means TSH is raised but thyroid hormone itself is still in the normal range. It is common, and its heart effects depend on how high the TSH is.
The best evidence pooled individual data from 11 studies and 55,287 adults. At a TSH of 4.5 to 6.9 mIU/L, the risk of coronary heart disease events was no higher than in people with normal thyroid function. At a TSH of 10 to 19.9 mIU/L, the risk was 89% higher.2
Lipids shift too, though less than in overt disease. A pooled analysis of placebo-controlled trials in subclinical hypothyroidism found that thyroid hormone treatment lowered total cholesterol by about 9% and LDL cholesterol by about 14%.3
What has not been shown is that treating a mildly underactive thyroid prevents heart attacks. A working group convened by the US National Heart, Lung, and Blood Institute pointed to significant knowledge gaps about how best to manage abnormal thyroid function in people with and without heart disease, and called for clinical trials that measure cardiovascular outcomes.4 Whether to treat a mildly raised TSH is a decision for you and your prescriber.
An overactive thyroid and atrial fibrillation
Too much thyroid hormone speeds the heart and makes the upper chambers more prone to atrial fibrillation, often shortened to AFib. AFib matters because it can lead to stroke and heart failure.
The link shows up even in mild cases. In pooled data from 52,674 adults, people with subclinical hyperthyroidism had a 68% higher risk of developing atrial fibrillation and a 29% higher risk of death from coronary heart disease. Risks were highest when TSH was below 0.10 mIU/L.5
The pattern extends into the normal range. Among adults with normal thyroid tests, those in the highest quarter of free T4 levels had a 45% higher risk of atrial fibrillation than those in the lowest quarter. A mildly underactive thyroid was not linked with atrial fibrillation in that analysis.6
If you take thyroid medication, do not adjust the dose yourself in response to palpitations or a lab result. Tell your prescriber.
Why thyroid status comes before a cholesterol decision
High LDL has many causes. Most are long term: genes, diet, weight and age. An underactive thyroid is different, because it is a cause that can be corrected, and LDL often falls when it is.
So when LDL is unexpectedly high, or has climbed without an obvious reason, clinicians commonly look at thyroid function early. It helps avoid two mistakes: treating a cholesterol number while missing the condition behind it, and blaming the thyroid for a cholesterol problem that has another cause.
This is an ordering of questions, not a rule about medication. Some people need both conditions managed, and that plan is made with their prescriber.
What a thyroid problem does not explain
A normal TSH takes the thyroid off the list. It says nothing about the rest of your risk.
The reverse also holds. If your thyroid is well controlled and your LDL is still high, the cholesterol needs to be assessed on its own terms, along with blood pressure, blood sugar, smoking and family history.
Lp(a), the inherited cholesterol particle, is a separate matter. The Thyroid review describes the effect of thyroid hormone on Lp(a) as still debated, with studies reporting either a decrease or no change.
Your next step
If you have a thyroid condition, it is worth knowing where your lipids stand once your thyroid levels are stable. Our guide on how to read a lipid panel goes through each line, and the ApoB guide explains the particle measure that a standard panel leaves out.
You can also answer the three quick questions in the screener on the Cardio Res-Q program page. If you want to know whether thyroid testing is part of an assessment, ask our team.
Are You a Candidate for Cardio Res-Q?
Three quick questions about the risk factors we screen for. Your answers stay between you and the clinic.
Frequently asked questions
Can an underactive thyroid cause high cholesterol?
Yes. With too little thyroid hormone the liver has fewer LDL receptors and clears LDL more slowly, so LDL cholesterol and ApoB rise. The effect is strongest in overt hypothyroidism and milder when only TSH is raised.
Will treating my thyroid fix my cholesterol?
It often improves it, and sometimes that is enough. Thyroid hormone replacement usually improves the lipid profile in overt hypothyroidism. But many people have more than one reason for high LDL, so cholesterol is normally rechecked once thyroid levels are stable. What happens next is a decision for you and your prescriber.
Can thyroid problems cause an irregular heartbeat?
Yes. An overactive thyroid, including a mild form with low TSH and normal thyroid hormone, is linked with a higher risk of atrial fibrillation. If you notice a fast or irregular pulse, tell your doctor. If it comes with chest pain, fainting or shortness of breath, call 911.
Does a slightly high TSH raise my heart risk?
At the mild end, the evidence says not much. In pooled data from more than 55,000 adults, coronary risk was not raised at a TSH of 4.5 to 6.9 mIU/L and was clearly raised at 10 mIU/L or above. Your clinician will read the number alongside your symptoms, age and other risk factors.
Related reading
This article is for general education and is not medical advice. Decisions about testing and medication should be made with your clinician. If you have chest pain or other symptoms of a heart attack, call 911.
