Thyroxine concentration in human milk

Thyroxine (T4) concentrations were measured by RIA in 37 milk samples from 19 healthy euthyroid mothers, obtained between 3-165 days postpartum. The mean milk T4 concentration in the first week postpartum was 0.38 ± 0.07 μg/100 ml (mean - SEM). The mean T4- concentrations between 8 and 4-8 days rose to 4.27 ± 0.50 μ/100 ml (mean ± SEM), and decreased to 1.11 ± 0.25 μg/100 ml (mean - SEM) after 50 days postpartum. The data suggest that human milk can provide a significant exogenous source of T4- to the infant. In the hypothyroid infant, the amount of 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 hypothyrcidism.