Unusual Magnitude and Variation of Plasma Protein-bound Iodine Values of Dairy Cattle

Summary Plasma PBI values of unusual magnitude and variations have been observed in the dairy herd at West Virginia University. The appearance of high PBI values is seasonal, occurring mostly in the late winter and spring. Plasma PBI follows a definite pattern with time by rising to a peak and then declining. A study of management practices revealed that the rise in PBI paralleled the feeding of grass silage to the dairy herd. High PBI levels were not associated with the feeding of excessive quantities of iodine compounds and were not characterized by high levels of plasma inorganic iodine. Fractionation revealed that, whereas some of the PBI was hormonally inactive, the remainder still indicated an unusually high level of circulating thyroid hormone. However, no symptoms of a pathological hyperthyroid condition could be observed in any of the animals studied. Although grass silage was not the sole cause of the elevation of PBI levels, it was capable of maintaining such levels when, normally, other conditions resulted in their decline.