A cDNA encoding transthyretin was cloned from the Pacific bluefin tuna (Thunnus orientalis). This cDNA contains a complete open reading frame encoding 151 amino acid residues. The deduced amino acid sequence is 81% and 55% identical to the gilthead seabream and common carp forms, respectively, and 33-39% to mammalian, reptilian, and amphibian forms. A 1.0-kb transcript was found in the the liver and ovary; the liver is the main source of this protein. Analysis of triiodo-Lthyronine (T3) and L-thyroxine (T4) binding demonstrated that both T3 and T4 bind to bluefin transthyretin. The binding activity of T3 for bluefin transthyretin is higher than that of T4. These results indicate that bluefin transthyretin acts as a transporter of thyroid hormones (THs) in the plasma, and plays an important role in the function of THs in target cells.