Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T₃ via transcriptional regulation. Two TR genes, α and β, encode four T₃-binding receptor isoforms (α1, β1, β2, and β3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T₃, transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T₃ target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T₃-dependent manner. In the absence of T₃, corepressors act to repress the basal transcriptional activity, whereas in the presence of T₃, coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRβ gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T₃. Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism.

Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin αvβ3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na⁺/H⁺ exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T₄ is an agonist at the plasma membrane without conversion to T₃. Tetraiodothyroacetic acid is a T₄ analog that inhibits the actions of T₄ and T₃ at the integrin, including angiogenesis and tumor cell proliferation. T₃ can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin αvβ3. Downstream consequences of phosphatidylinositol 3-kinase activation by T₃ include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase.

Thyroid hormone, chiefly T₃ and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T₄ and rT₃, but not T₃, is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth …