[Cellular mechanisms of thyroid hormone action].

Thyroid hormones affect cardiac myocytes as well as the smooth muscle and endothelial cells of the vascular wall. Free 3,53'-L-triiodothyronine (FT3) and its specific nuclear receptor modulate the transcription of various proteins, principally those involved in the myocyte contractile apparatus (myosin heavy chains), and the regulation of intracellular calcium flux (sarcoplasmic reticulum Ca2+ATPase). Thyroid hormones also have non-genomic effects that work rapidly, complement the effects described above, and are related to alterations in the properties of many channels and membrane receptors, especially in the sinoatrial mode. Thyroid hormones also affect the smooth muscle and endothelial cells of the vascular walls and reduce systemic vascular resistance. These effects on cardiac and vascular cells globally explain the cardiac manifestations (especially the inotropic and chronotropic effects) observed during dysthyroidism, particularly in hyperthyroidism where they are often in the forefront (positive inotropic and chronotropic effects).