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.

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.

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.

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.

When high thyroid hormone levels are caused by taking too much thyroid medication (exogenous thyrotoxicosis), the underlying biochemical changes differ from those caused by an overactive thyroid gland (endogenous hyperthyroidism). Because of these differences, relying only on TSH levels to assess risk can lead to incorrect conclusions in one condition versus the other.