Levothyroxine Palpitations: Causes, Risks, and What to Do
When you take levothyroxine, a synthetic form of the thyroid hormone T4 used to treat hypothyroidism. Also known as Synthroid, it helps restore normal metabolism, energy, and heart function — but only if the dose is right. If your heart starts racing, fluttering, or pounding for no reason, it’s often a sign your body is getting too much of this hormone. Levothyroxine palpitations aren’t rare — they’re a red flag that your dose might be too high, or your body isn’t handling it well.
Thyroid hormones directly affect your heart. Too much levothyroxine speeds up your heart rate, increases blood pressure, and can trigger atrial fibrillation, especially in older adults or people with existing heart conditions. It’s not just about feeling anxious — your heart is literally working harder than it should. Studies show that up to 1 in 5 people on levothyroxine report palpitations, and in many cases, it’s not anxiety — it’s pharmacology. Your doctor should check your TSH levels regularly, but if you’re having symptoms, don’t wait for your next appointment. Palpitations can be a sign of overdose, a condition where thyroid hormone levels exceed safe limits. Also known as thyrotoxicosis, it can lead to serious complications like stroke or heart failure if ignored. Other factors like switching brands, taking it with food, or interactions with other meds (like iron or antacids) can also cause sudden spikes in hormone levels, even if your dose hasn’t changed.
It’s not just about the pill — it’s about how your body reacts. Some people metabolize levothyroxine faster, others slower. Age, weight, liver function, and even gut health play a role. If you’ve recently started a new supplement, changed your diet, or had surgery, your thyroid needs might have shifted. Don’t assume your current dose is still perfect. The goal isn’t just to get your TSH into the "normal" range — it’s to feel normal. If your heart is racing, you’re sweating more than usual, or you’re losing weight without trying, your body is telling you something’s off.
What comes next matters. Your doctor might lower your dose, switch you to a different brand, or check for other causes like adrenal issues or heart disease. But you don’t have to wait. Track your symptoms: when do palpitations happen? After taking the pill? On an empty stomach? With caffeine? Write it down. Bring it to your appointment. The right dose isn’t one-size-fits-all — it’s personalized, and it’s worth fighting for.
Below, you’ll find real-world insights from people who’ve been there — how they figured out what was causing their heart to race, what worked, and what didn’t. No fluff. No guesswork. Just clear, practical info to help you take control.
