Thyroid Medication and Heart Rate: What You Need to Know

When you take thyroid medication, a drug used to replace or regulate thyroid hormone in people with underactive or overactive thyroid glands. Also known as thyroid hormone replacement, it helps restore normal metabolism, energy, and body temperature—but it can also change your heart rate in ways you might not expect. Too little hormone leaves you tired and slow; too much pushes your heart into overdrive. That’s why heart rate changes are one of the most common signs your dose might be off.

Levothyroxine, the most prescribed thyroid medication, a synthetic form of the T4 hormone. Also known as Synthroid, it’s usually safe when dosed right—but even small increases can raise your resting heart rate by 10 to 20 beats per minute. If you’re suddenly feeling your heartbeat in your chest, getting winded climbing stairs, or noticing palpitations, it’s not just anxiety. It could be your thyroid dose. On the flip side, if your heart rate stays too low—under 60 bpm—you might still be under-treated. Both extremes stress your heart over time. People with existing heart conditions, like atrial fibrillation or coronary artery disease, are especially sensitive. A study from the Journal of Clinical Endocrinology & Metabolism found that older adults on levothyroxine with heart disease had a 30% higher risk of irregular heartbeat if their TSH levels dropped too low.

Hypothyroidism, a condition where the thyroid doesn’t make enough hormone, often leading to slow heart rate, fatigue, and weight gain. Also known as underactive thyroid, it’s usually treated with thyroid medication—but getting the balance right takes time. Your doctor won’t just check your TSH levels—they should also monitor your pulse, blood pressure, and symptoms over weeks, not days. Too many people get their dose adjusted too fast, then wonder why they’re jumpy or dizzy. Slow, steady changes are safer. And if you’re on other meds—like beta-blockers for high blood pressure or anxiety—those can mask the signs your thyroid dose needs tweaking. That’s why tracking your own heart rate at home, even just once a day, can be more useful than waiting for your next appointment.

Thyroid medication isn’t just about energy or weight. It’s a direct line to your heart. Whether you’re new to treatment or have been on it for years, your pulse is one of the clearest signals your body gives you. If something feels off, don’t ignore it. The right dose can make you feel like yourself again. The wrong one? It can leave you tired, shaky, or worse.

Below, you’ll find real patient experiences, expert advice on dosing, and warnings about dangerous interactions—everything you need to keep your thyroid and your heart in sync.