The thyroid hormone (TH; 3,3',5,5'-tetra-iodothyronine and 3,3',5-triiodothyronine) regulates growth, development, and critical metabolic functions. Thyroid diseases are among the most prevalent group of metabolic disorders in the Western world. TH exert's effects through complex biological pathways, which offer a wealth of opportunities to pharmacologically intervene in TH signalling at numerous steps. These include biosynthesis, cell-specific uptake or export (involving L-type amino acid transporter, organic anion transporter, organic cation transporter, or multidrug resistance transporter), as well as nuclear targeting and actions (the latter including TH receptor binding and histone acetylation/deacetylation). Such processes represent potentially important pharmacological targets for the design of novel or improved therapies for TH disorders, obesity, and cardiovascular diseases. (C) 2002 Elsevier Science Inc. All rights reserved.