Factors Affecting the Secretion of Iodine131 into Milk of Lactating Goats

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Abstract
Radioactive iodine (I131) was injected into 8 lactating goats and its relative concentration in the iodide and protein-bound iodine (PBI) fractions of milk and blood serum was compared. About 92% of the I131 in milk occurred as iodide and the remainder was found in the PBI fraction. During 70 hr. following I131 injection the iodide concentration in milk of control animals was more than ten times that in blood serum, indicating an active process in mammary tissue for the secretion of iodide. Milk: serum iodide concentration was not affected by administration of 5.0 g thiouracil or 50.0 mg of nonradioactive KI daily. The concentration gradient was abolished, however, by injecting 1.0 g daily of KSCN. None of the drug treatments had a discernible effect on the PBI fraction of milk. Excluding values for the thiocyanate group (where milk iodide secretion was blocked), a 0.795 correlation (P = 0.07) was found between milk iodide: serum iodide ratio and daily milk yield. In a preliminary experiment, this correlation was 0.913 (P = 0.01). Thus, it is pointed out that in high-producing animals total iodide secretion into milk is increased not only because of the greater volume of milk produced but also because of the higher iodide concentration gradient maintained in highly active mammary tissue. This loss of iodide in milk could indirectly become a limiting factor on milk production by reducing the amount of iodide available for the formation of thyroid hormone.