Thyroxine concentration in human milk.

Thyroxine (T4) concentrations were measured by RIA in 37 milk samples from 19 healthy eutryroid mothers, obtained between 3-165 days postpartum. The mean milk T4 concentration in the first week postpartum was 0.38 +/- 0.07 microgram/100 ml (mean +/- SEM). The mean T4 concentrations between 8 and 48 days rose to 4.27 +/- 0.50 microgram/100 ml (mean +/- SEM), and decreased to 1.11 +/- 0.25 microgram/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 detrimetal effects of hypothyroidism.