Thyroxine of human and bovine milk origin: possible bearing on congenital hypothyroidism

Thyroxine (T4) concentrations were measured by RIA in 89 milk samples, obtained between 3-165 days postpartum from 40 healthy euthyroid mothers. The mean milk T4 conc. in the first week postpartum was 0.4 ± 0.1 μg/100 ml (n=9). The mean T4 concentrations between 8 and 48 days postpartum rose to 2.6 ± 0.4 μg/100 ml (n=57), and decreased to 1.28 ± 0.3 μg/100 ml (n=23) after 50 days postpartum. T4 concentrations in bovine (n=15) milk samples were less than 0.4 μg/100 ml. The data suggest that milk of humna, but not bovine, origin can provide a significant exogenous source of T4 to the infant, In the hypothyroid infant, T4 in human milk may delay clinical recognition of the disease. Although this exogenous source of T4 may alleviate the disease, it is insufficient to prevent the detrimental effects of hypothyroidism.