protein-bound

Said of a chemical bonding of calcium, iodine, iron, copper and other electrolyte ions in the blood to plasma proteins. The bound electrolyte is not immediately available biologically nor is it as vulnerable to sudden loss.

• p.-bound iodine (PBI) test — determination of thyroid function by measuring the amount of iodine contained in compounds bound to plasma proteins. It has been largely replaced by radioimmunoassay for the thyroid hormones thyroxine (T₄) and tri-iodothyronine (T₃).