Iodine absorption, excretion, recycling, and tissue distribution in the dairy cow.

Research on iodine metabolism was reviewed with special reference to cattle and milk secretion. Iodine metabolism in the fetus and thyroid activity as related to milk secretion and iodine feeding have also been considered. Between 70 and 90% of dietary iodine is absorbed directly from the bovine rumen, reticulum, and omasum. In the abomasum, rate of iodide secretion is approximately 18 times the rate of absorption. Circulating thyroxine is not secreted into the abomasum but enters the small intestine in the bile. Most of the iodide secreted into the abomasum is reabsorbed from the small and large intestines; absorption of thyroxine averages less than 10%. The thyroid usually contains an amount of iodine equal to the daily dietary intake; extrathyroidal tissues concentrate only .006 to .04% as much radioiodine per unit weight as the thyroid. At normal iodine intake (less than 10 mug/kg body weight daily), iodine losses average 30% of the daily intake in feces, 40% in urine, and 8% in milk. Milk iodine is influenced by amount and chemical form of dietary iodine, stage of lactation or milk yield, seasonal effects, goitrogens, and thyroid status. Cows secrete less iodine into milk than most other species and have an efficient iodine recycling system via the gastrointestinal tract which conserves iodine and can protect them against low dietary iodine.