Thyroxine (by competitive protein binding analysis) in human and cow milk and in infant formulas.

In 45 mothers of full term infants the level of thyroxine (T4) in serum (45 samples) and unskimmed milk (92 samples) was estimated with the aid of competitive protein binding analysis. On the 3rd day after delivery the T4 level in serum was found to be 11.1 +/- 0.4 mug/100 ml, while that in milk was 1.3 +/- 0.3 mug/100 ml. During following days of lactation the T4 concentration in sera decreased. In contrast, however, a gradual increase of the T4 content in milk was observed. The serum and milk levels equalized during the third week post partum (7.3 +/- 1.3 and 7.6 +/- 1.4 mu6/100 ml, respectively). After the 25th day the T4 level in milk was found to be higher than in sera from the same women (paired test: P less than 0.005). Values of 12.9 +/- 1.3 in milk and 8.1 +/- 0.7 mug/100 ml in sera were found in the second month of lactation. A negative correlation between serum and milk T4 level was found during the early lactation (3-20 days), while after 20th day this correlation was positive. Very low and almost undetectable levels of T4 were found in cow's milk and infant formulas derived from it.