Thyroid Hormones and Co-workers: An Overview

The hypothalamus secretes the thyroid-releasing hormone (TRH) that induces the pituitary gland to release the thyroid-stimulating hormone (TSH) which stimulates thyroid follicular cells to release the thyroid hormones (THs), thyroxine (T4), and triiodothyronine (T3). The process of synthesizing T3 and T4 hormones involves various enzymatic steps, starting with the iodination of L-tyrosine residues present in the protein thyroglobulin. Thyroid hormones are released into the bloodstream, where they bind to thyroid hormone distributor proteins (THDPs) which transport them in the circulation. The conversion of T4 to T3 (the more biologically active hormone) in target tissues is facilitated by selenoprotein enzymes known as deiodinases. THs can bind to different molecules located on the plasma membrane, such as integrin αvβ3, through which they exercise regulatory non-genomic control. Nevertheless, most of thyroid hormone's actions are mediated intracellularly by binding to thyroid hormone receptors (TRs). Thyroid hormone receptors act as ligand-dependent transcription factors, Thyroid hormone receptors activate thyroid hormone response elements on gene promoters through canonical signaling. Thyroid hormones mediate several critical physiological processes including organ development, cell differentiation, metabolism, and cell growth and maintenance.