The effect of thyroxine and thiouracil on the composition of milk. 3. The concentration of the major constituents and of some of the water-soluble vitamins in cow's milk.

When lactating animals are treated with thyroxine, milk yield is almost invariably markedly stimulated (Graham, 1934a, b; Jack & Bechdel, 1935; Folley & White, 1936; Herman, Graham & Turner, 1938; Smith & Dastur, 1940; Owen, 1948a; Chanda & Owen, 1951). Usually the yield of fat is stimulated to a greater extent than that of the milk so that an increase in fat percentage results. This latter effect is, however, not always observed. In some experiments the solids-not-fat of the milk increase (Folley & White, 1936), but this again is not a constant finding (Archibald, 1945). The changes in the total nitrogen content of the milk caused by iodinated casein or thyroxine are also highly variable. Thus Ralston, Cowsert, Ragsdale, Herman & Turner (1940) reported a decline in the total nitrogen content of the milk. On the other hand, Van Landingham, Hyatt & Weakley (1946), Hibbs & Krauss (1947), Owen (1948a) and Chanda & Owen (1951) found no change in the total nitrogen content or in the partition ofnitrogen in the milk. It came to be generally believed that, apart from its effect on fat, thyroxine caused only minor changes in the composition of the milk. Owen (1948a), however, found a significant increase in the phosphorus content of the milk without any concomitant change in either the nitrogen or the calcium contents. Chanda & Owen (1951) found that the goitrogenic substance thiouracil caused a decrease in the total phosphorus ofmilk without any change in its nitrogen content, and in the same experiments they showed that thyroxine increased the ester phosphorus and lipid phosphorus while decreasing the inorganic phosphorus in cow's milk. They also found that thiouracil produced changes in the partition of phosphorus which were the reverse of those produced by thyroxine. Changes in phosphorus partition, whether produced by thyroxine or thiouracil, were inversely correlated with changes in milk phosphatase. In the present paper the effects of thyroxine and thiouracil on the partition of aneurin in relation to changes in the phosphatase titre of the milk are described. The simultaneous changes produced in the concentrations of some of the major constituents of milk and in the concentrations of certain minerals and water-soluble vitamins were also followed. EXPERIMENTAL