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.

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.

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.

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.

In people with thyrotoxicosis caused by taking too much thyroid hormone medication, the brain's control system for thyroid hormone levels behaves differently than in people whose thyroid gland overproduces hormones on its own. Specifically, TSH stays low even when thyroid hormone levels are higher than normal.