What are the effects of thyroid hormone T3 on the heart, and what is notable about its mechanism?

• A. Increases inotropy and lusitropy
• B. Decreases inotropy and lusitropy
• C. Increases heart rate only
• D. Has no direct effect on the heart

Answer: (A)

The main idea is that T3 directly makes the heart beat more forcefully and recover faster, meaning positive inotropy and positive lusitropy. This happens through a genomic mechanism: T3 enters cardiac myocytes and binds nuclear thyroid hormone receptors, changing the transcription of genes that govern calcium handling and contractile proteins. As a result, there is increased expression of proteins like SERCA2a and favorable shifts in myosin heavy chain isoforms, which improve calcium cycling and the strength of contraction. These changes also heighten the heart’s responsiveness to catecholamines, reinforcing the positive effects on both contraction and relaxation. Because this relies on gene expression, the effects are sustained rather than just immediate, setting T3’s action apart from rapid, non-genomic signals.