Too much or too little thyroid medicine can hurt your heart
Association between Over- and Under-Replacement with Thyroid Hormone and Incident Heart Failure
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
Mild over-replacement (TSH 0.1–0.5) showed no increased heart failure risk — and possibly lower risk.
Most clinicians and patients assume any TSH below 1.0 is dangerous and must be corrected — but this massive study shows no harm, and even a hint of benefit.
Practical Takeaways
If you're on thyroid hormone, ask your doctor for your last 3 TSH and FT4 results — and track if you’ve been consistently above TSH 5.5 or below 0.1 for over a year.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
Mild over-replacement (TSH 0.1–0.5) showed no increased heart failure risk — and possibly lower risk.
Most clinicians and patients assume any TSH below 1.0 is dangerous and must be corrected — but this massive study shows no harm, and even a hint of benefit.
Practical Takeaways
If you're on thyroid hormone, ask your doctor for your last 3 TSH and FT4 results — and track if you’ve been consistently above TSH 5.5 or below 0.1 for over a year.
Publication
Journal
The Journal of clinical endocrinology and metabolism
Year
2025
Authors
Josh M. Evron, Brandon Moretti, Richard Evans, J. Burns, Scott L. Hummel, Nazanene H. Esfandiari, S. Hawley, M. Haymart, Maria Papaleontiou
Related Content
Claims (6)
People with consistently elevated TSH levels above 5.5 mIU/L due to insufficient thyroid hormone replacement have a higher risk of developing heart failure over time, and the risk increases further as TSH levels rise above that threshold.
Adults on thyroid hormone replacement therapy who consistently have very high TSH levels (above 20 mIU/L) are at significantly higher risk of developing heart failure over time compared to those with normal TSH levels, with the risk increasing substantially after five years.
In adults taking thyroid hormone medication, consistently having too much hormone in the blood—measured by very low TSH or high free thyroxine—is linked to a higher chance of developing heart failure.
Long-term use of too little or too much thyroid hormone medication is linked to a higher risk of developing heart failure, with the risk increasing the longer the imbalance lasts. After five years, too little hormone raises the risk more than too much.
Giving synthetic thyroid hormone externally raises levels of thyroid hormone in the blood and makes an existing overactive thyroid condition more severe.