The Study
Association between Over- and Under-Replacement with Thyroid Hormone and Incident Heart Failure
This study looked at a lot of people who took thyroid medicine and noticed that those with too little or too much hormone were more likely to get heart failure later. But it didn’t change anyone’s medicine—it just watched what happened. So we can’t say the hormone levels caused the heart problems, just that they often happened together.
Analysis score
Maximum 72 for a cohort study.
Where the score came from
People taking thyroid hormone pills need the right dose—too little or too much can make their heart work harder and increase heart failure risk over time.
Where does this study sit?
Reviews of RCTs (Meta-analyses)
Max 100Randomized Trials
Max 90Reviews of Cohort Studies
Max 85Cohort Studies
Max 72Reviews of Case-Control Studies
Max 63Case-Control Studies
Max 58Cross-Sectional & Case Series
Max 50Expert Opinion
Max 559 / 100
Quality score
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
Key takeaways
Summary
Based on the study abstract and findings.
- 1Yes—this means staying in the right hormone range is as important for heart health as controlling blood pressure or cholesterol.
- 2Too little hormone (TSH >20): 93% higher heart failure risk; too much (TSH <0.1): 6% higher risk; very high FT4 (>1.9 ng/dL): 23% higher risk; after 5 years of too little, risk jumps 5.8 times.
Score breakdown, methodology, conflicts of interest, evidence analysis & raw study data
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)
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.
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.
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.
When thyroid hormone levels are slightly above normal, heart failure risk does not increase and may be slightly lower; when levels are much higher, heart failure risk increases. This suggests heart failure risk is lowest at moderate thyroid hormone levels and rises at both lower and higher extremes.
Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.