Relatively little is known about thyroid hormone-binding proteins in fish and, until recently, the thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), had only been found in fish plasma bound to albumin and lipoproteins. Recently, transthyretin (TTR) was cloned in a teleost fish, the sea bream (sb); it is composed of 130 amino acids and shares 47-54% sequence similarity with other vertebrate TTR and binds preferentially T3. Homology modelling of sbTTR based upon the crystallographic structure of TTR in human, rat and chicken reveals similar monomer-monomer and dimer-dimer interfaces and a conserved tetrameric structure. In sbTTR, a single amino acid substitution in the thyroid hormone binding site (Ser 117 in human by Thr in sea bream) may explain the higher affinity of this tetramer for T3 rather than T4. The principal site of production of TTR in the sea bream is the liver but transcripts are also present in the intestine, brain, skin, heart, skeletal muscle, kidney, testis, gills and pituitary (in descending order of abundance). The function of TTR in fish remains to be studied but we have recently carried out studies which suggest it may be involved in TH balance during food shortage.