Content of thyroid hormones in human and animal milk and in cow milk based infant formulas.
The concentration of thyroxine (T4), triiodothyronine (T3), thyroxine binding globulin (TBG) as well as thyroxine binding to proteins (TBP) and free thyroxine index (FTI) were measured in human milk. In addition, the concentration of T4 in milk of rats, rabbits and cows and in cow milk based infant formulas was measured. It was found that the concentration of T4 and T3 in human milk or that of T4 in rat and rabbit milk is much higher than in corresponding colostrum. The concentrations of TGB and also TBP and FTI in human milk were much less than in blood serum. In cow milk and in cow milk based products used for infant nutrition the concentration of T4 was either undetectable or only traces were found. The data suggest that the hormonal interrelation between the mother and the fetus should operate even in the postnatal period via the excretion of some hormones by milk.