Thyroxine-binding globulin cleavage in cord blood.

Thyroxine-binding globulin, a member of the serine protease inhibitor superfamily of proteins (serpins), releases T(4) on cleavage by polymorphonuclear elastase. Such cleavage, previously shown to occur during sepsis and with an exogenous inflammatory stimulus, is now demonstrated in the cord blood of normal babies and appears to be part of a physiological inflammatory response in the newborn. In association with the neonatal TSH surge, thyroxine-binding globulin cleavage is likely to contribute to an increased flux of T(4) to neonatal tissues at a time when T(4)-sensitive morphogenic and biochemical changes are occurring.