Yes, an underactive thyroid can cause heart palpitations, even though most people associate a racing heart with an overactive thyroid. When your thyroid isn't producing enough hormones, your heart may compensate by pumping harder with each beat, creating a pounding sensation. Additionally, low thyroid hormone levels can make your heart's electrical system more irritable, leading to skipped beats or fluttering sensations that feel alarming but are often harmless .

How Does an Underactive Thyroid Affect Your Heart?

Your thyroid gland produces two main hormones, Thyroxine (T4) and Triiodothyronine (T3), that act as chemical messengers throughout your body. These hormones tell your cells how much energy to use and how fast to work. Your heart is one of the thyroid's most sensitive targets because thyroid hormones directly influence three critical cardiac functions :

• Heart Rate Control: Thyroid hormones regulate the pacemaker cells in your heart that determine how many times it beats per minute.
• Force of Contraction: These hormones affect how strongly your heart muscle squeezes to pump blood throughout your body.
• Blood Vessel Tension: Thyroid hormones help regulate how relaxed or tense your blood vessels are, which directly impacts your blood pressure.

When your thyroid is underactive, it produces fewer of these vital hormones. This means the signals sent to your heart become muffled or slow. While you might expect this to only result in a slower heart rate, your body is far more complex. When one system slows down, other systems may try to compensate, or the heart's rhythm itself may become irritable, leading to the palpitations you feel .

Why Does a Slow Thyroid Cause a Racing or Fluttering Heart?

The connection seems counterintuitive, but there are several clinical reasons why hypothyroidism triggers palpitations. In many cases of hypothyroidism, your heart rate actually slows down. Because your heart is beating fewer times per minute, it compensates by pumping more blood with each individual beat to ensure your organs receive adequate oxygen. This results in a pronounced "pounding" sensation, especially noticeable when you're lying down or in a quiet room where you can hear each heartbeat .

Beyond the pounding sensation, low thyroid hormone levels can make your heart's electrical system more "irritable." This irritability can lead to premature contractions, often called ectopic beats. These feel like your heart has "skipped a beat" or delivered a sudden "thump" in your chest. While these beats are often harmless, they can be deeply distressing if they occur frequently .

Even people in a category called "subclinical hypothyroidism" experience this effect. In subclinical hypothyroidism, your Thyroid Stimulating Hormone (TSH) is slightly elevated, but your actual thyroid hormone levels (T4 and T3) remain within the "normal" range according to standard NHS guidelines. Research has shown that even these subtle shifts in thyroid function can increase the risk of rhythm disturbances .

How to Identify and Manage Thyroid-Related Palpitations

• Track Your Symptoms: Keep a detailed log of when palpitations occur, what you were doing, what you ate, and how you felt emotionally. Note whether the sensation is a pounding, fluttering, or skipped beat, and how long episodes last.
• Get Comprehensive Thyroid Testing: Ask your GP for a full thyroid panel that includes TSH, Free T4, and Free T3 levels, not just TSH alone. This gives a more complete picture of your thyroid function.
• Review Your Medications: If you're already taking thyroid medication like Levothyroxine, your palpitations might indicate your dose is too high. Work with your doctor to adjust your prescription rather than changing it yourself.
• Address Nutrient Deficiencies: Hypothyroidism can impair nutrient absorption, leading to low iron or vitamin B12, both of which independently cause palpitations. Ask your doctor to test for these deficiencies.
• Seek Emergency Care When Needed: If you experience sudden, severe palpitations accompanied by chest pain, significant shortness of breath, dizziness, or fainting, call emergency services immediately rather than waiting for an appointment.

The Medication Complication: When Treatment Causes Palpitations

There's an important twist for people already being treated for hypothyroidism. If you're taking Levothyroxine, a synthetic version of the T4 hormone, your palpitations might actually signal that your dose is too high for your current needs. This situation is called "iatrogenic hyperthyroidism," meaning your medication has pushed you from being underactive to being slightly overactive. When you have too much thyroid hormone in your system, your heart rate speeds up and your risk of palpitations or even atrial fibrillation (an irregular, fast heart rhythm) increases significantly .

If you're on thyroid medication and start feeling "revved up," shaky, or notice your heart racing, it's essential to speak with your GP. They can arrange a blood test to check your hormone levels and adjust your prescription if necessary. Never adjust your medication dosage yourself based on a private test result; always work in partnership with a medical professional .

What Should You Do If You Suspect Your Thyroid Is Causing Palpitations?

Your first step should always be consulting your NHS GP. Palpitations can stem from many causes: stress, caffeine consumption, menopause, underlying heart conditions, or medication side effects. Your GP can perform an initial physical examination, possibly an ECG (Electrocardiogram) to check your heart's electrical activity, and run standard thyroid function tests. It's important to rule out any underlying cardiac issues before focusing solely on the thyroid .

The key takeaway is that an underactive thyroid is a legitimate but often overlooked cause of heart palpitations. By understanding how thyroid hormones influence your heart's function, you can have more informed conversations with your healthcare provider and work toward identifying the root cause of your symptoms rather than treating them in isolation.