The T3 receptor, also known as thyroid hormone receptor (TR), is a protein that belongs to the nuclear receptor superfamily and plays a crucial role in mediating the actions of thyroid hormones (triiodothyronine, or T3). These receptors are encoded by two separate genes, TRα and TRβ, which express different isoforms referred to as TRα1, TRα2, TRβ1, and TRβ2.

As a transcription factor, the T3 receptor controls gene expression by binding to specific DNA sequences known as thyroid hormone response elements (TREs) in the promoter region of target genes. Upon binding to T3, the receptor undergoes a conformational change, enabling it to recruit coactivator or corepressor proteins to modulate gene transcription. This process ultimately leads to the regulation of various physiological processes, including metabolism, development, growth, and differentiation.

Abnormalities in T3 receptor function can result in thyroid hormone resistance, a rare genetic disorder characterized by reduced sensitivity to thyroid hormones. This condition can lead to symptoms such as goiter, growth retardation, delayed bone maturation, and impaired cognitive development. Additionally, disruption of T3 receptor signaling has been implicated in various diseases, including cancer, diabetes, and cardiovascular disorders.

In summary, the T3 receptor is a key mediator of thyroid hormone actions, modulating gene transcription to regulate diverse physiological processes.