Breast milk thyroxine and not cow's milk may mitigate and delay the clinical picture of neonatal hypothyroidism.

Thyroxine concentration was measured in human milk and Cow's milk products by a specific radioimmunoassay. The mean (+/- S.E.M.) milk T4 concentration during the first 5 days postpartum was 0.7 +/- 0.3 microgram/dl (n = 11). The mean T4 concentration between 6-49 days postpartum rose to 3.1 +/- 0.2 microgram/dl (n = 108), falling after 50 days to a mean of 1.4 +/- 0.2 microgram/dl (n = 39). The mean (+/- S.E.M.) T3 concentration in breast milk in the first 50 days postpartum was 386 +/- 17 ng/dl (n = 56). T4 concentration in cow's milk products was less than 0.3 microgram/dl. Thyroxine concentration in 24 hours breast milk collection ranged from 0.7 to 7.7 microgram/dl and the total T4 in this milk ranged from 0.7 to 28 microgram/day. These data suggest that milk of human but not bovine origin may provide a significant exogenous source of T4 to the premature infant. This amount of exogenous T4 which is insufficient in preventing the proceeding of neonatal hypothyroidism, may delay the clinical recognition of this disorder. This once again emphasized the importance of early screening for neonatal hypothyroidism.