Slow Heart Rate (Bradycardia)
An underactive thyroid can slow the heart rate — a pattern called bradycardia. Because thyroid hormone helps set the pace of the heart, low levels can reduce both heart rate and the strength of contraction.
Bradycardia has many other causes, including medications, athletic fitness and heart conduction problems, so it should not be assumed to be thyroid-related without assessment.
Last medically reviewed: 15 November 2025
Key points
- Hypothyroidism can cause a slow heart rate, typically alongside fatigue and cold intolerance.
- Some medications, especially beta-blockers, commonly slow the heart.
- A very slow pulse with dizziness or fainting needs urgent assessment.
- Treating hypothyroidism usually restores a normal heart rate over time.
The thyroid–heart link
Thyroid hormone increases the heart’s responsiveness to adrenaline and influences the conduction system. When levels are low, the heart may beat more slowly and contract less forcefully.
Severe hypothyroidism can also cause fluid around the heart and changes in blood pressure.
Other causes
Bradycardia is common in fit athletes and during sleep. Medications such as beta-blockers, calcium channel blockers and digoxin commonly slow the heart. Heart conduction disease, electrolyte disturbances and hypothermia are other causes.
Symptoms such as dizziness, fainting, breathlessness or chest pain suggest the slow rate is causing problems.
Assessment
Assessment includes pulse and blood pressure measurement, an ECG and thyroid function tests. Medication review is important. Further cardiac testing may be arranged if a conduction problem is suspected.
Management
If hypothyroidism is the cause, levothyroxine usually restores the heart rate over weeks to months. In older people or those with heart disease, treatment is started cautiously.
Symptomatic bradycardia from a heart cause may need medication changes or a pacemaker.
When to seek urgent medical help
- Fainting, near-fainting or severe dizziness.
- Chest pain, breathlessness or confusion.
- A very slow pulse with low blood pressure.
This guidance does not replace emergency services. If you are seriously unwell, seek urgent medical care immediately.
Frequently asked questions
Can hypothyroidism cause a slow heart rate?
Yes — it can slow the heart rate and reduce the force of contraction, usually alongside other symptoms such as fatigue and cold intolerance.
Will treatment speed up my heart?
Usually the heart rate returns towards normal as thyroid hormone levels are corrected, though this can take weeks.
Is a slow heart rate dangerous?
It can be, especially with dizziness or fainting. Symptomatic bradycardia needs prompt assessment.
Can hyperthyroidism cause a slow heart rate?
No — it typically causes a fast heart rate. A slow rate in hyperthyroidism may suggest a separate heart or medication issue.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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