Can Hypothyroidism Cause a Fast Heart Rate?

Hypothyroidism, or underactive thyroid, typically slows down bodily functions rather than speeding them up. This condition occurs when the thyroid gland fails to produce sufficient thyroid hormones, which play a crucial role in regulating metabolism, energy levels, and cardiovascular function. As a result, individuals with untreated hypothyroidism generally experience a decrease in metabolic rate, leading to symptoms such as fatigue, cold intolerance, dry skin, weight gain, and slowed heart rate (bradycardia).

Understanding the Thyroid-Heart Connection

Thyroid hormones—particularly T3 (triiodothyronine) and T4 (thyroxine)—directly influence heart function by increasing myocardial contractility, cardiac output, and heart rate. When these hormone levels are low due to hypothyroidism, the heart tends to beat more slowly. This is why a fast heart rate is not a typical symptom of untreated hypothyroidism; in fact, the opposite is usually true.

When Might a Hypothyroid Patient Experience a Rapid Heartbeat?

While bradycardia is common in hypothyroid patients, there are specific scenarios where an elevated heart rate may occur. It's important to distinguish between physiological responses and potential complications:

1. Excessive Levothyroxine Dosage

One of the most common reasons for a fast heart rate in someone diagnosed with hypothyroidism is overtreatment with synthetic thyroid hormone—levothyroxine sodium. If the prescribed dose exceeds what the body needs, it can lead to a state resembling hyperthyroidism, known as drug-induced thyrotoxicosis. Symptoms include palpitations, anxiety, tremors, sweating, and tachycardia (heart rate over 100 bpm). Regular monitoring of TSH, free T4, and clinical symptoms is essential to maintain proper dosing.

2. Coexisting Cardiac Arrhythmias

In some cases, a rapid heartbeat may stem from underlying heart conditions unrelated to thyroid status. Hypothyroid patients—especially older adults or those with pre-existing cardiovascular disease—may develop arrhythmias such as atrial fibrillation, premature ventricular contractions (PVCs), or supraventricular tachycardia (SVT). These conditions require thorough cardiac evaluation, including ECG, Holter monitoring, and consultation with a cardiologist.

When to Seek Medical Attention

If a person with hypothyroidism notices persistent palpitations, shortness of breath, dizziness, or unexplained increases in heart rate, they should seek prompt medical assessment. These symptoms could indicate improper medication dosing, progression of heart disease, or other endocrine imbalances that need timely intervention.

In summary, untreated hypothyroidism does not cause a fast heart rate—on the contrary, it often leads to a slower-than-normal pulse. However, once treatment begins, careful management is key to avoiding overcorrection. Patients should maintain regular follow-ups with their healthcare provider to ensure optimal thyroid hormone levels and overall cardiovascular health.