Biochemistry. A serum transport protein mainly secreted by the liver. Transthyretin transports the thyroid hormones thyroxine, triiodothyronine, and also the retinol-binding protein in the blood. It is an excellent indicator of liver function and nutritional status. Transthyretin level in the blood is 18 times higher than in the cerebrospinal fluid. It has a short biological half-life of 48-72 hours.