Thyroid hormone-binding protein, with a much higher binding affinity for triiodothyronine (T3) than for thyroxine (T4). Probably transports triiodothyronine from the bloodstream to the brain. Homotetramer. Dimer of dimers. In the homotetramer, subunits assemble around a central channel that can accommodate two ligand molecules. Interacts with RBP4 (By similarity). Detected in plasma (at protein level). Expressed during metamorphosis in tadpole liver but not in tadpole brain, nor adult liver. Expressed in tadpoles from premetamorphic stage X, increasing until the end of prometamorphic stage (stage XX). Expression then declines during metamorphic climax stages (stages XXI-XXV), becoming undetectable at stage XXV and in adults. The N-terminus strongly influences thyroid hormone-binding properties. A number of compounds can compete with and disrupt triiodothyronine (T3)-binding. Binds the synthetic estrogen diethylstilbestrol (DES) with the same affinity as T3. Belongs to the transthyretin family.