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Protein bound iodine (PBI) and total cholesterol levels in serum were determined from 589 arterial samples of healthy dairy cows in different stages of lactation during a period of 18 months. PBI values varied from 2.10 to 6.29 ɣ %, with approximately 79 % of the cases occurring between 3 and 5 ɣ % and the average PBI level being 3.935 ɣ %. Cholesterol values varied from 37.6 to 494.4 mg %, with approximately 66 % of the cases between 100 and 300 mg % and the average colesterol level being 197.43 mg %. The stage of lactation had a greater influence on total cholesterol level than on PBI level. PBI level was lowest one month post partum (3.65 ± 0.08 ɣ %), but remained rather constant from the 5th to 13th month post partum, varying between 3.96 ± 0.08 and 4.11 ± 0.08 ɣ %. It was assumed that during the early stages of lactation the disappearance of thyroid hormones from the serum may be somewhat faster than their synthesis within the thyroid gland, and after an adaptation period in the later stages of lactation, the thyroid gland is able to maintain an optimal PBI level in the serum. The average cholesterol level of 177 mg % one month post partum was relatively low in regard to milk production compared with cholesterol levels obtained at later stages of lactation. The relatively low cholesterol values in the early part of lactation may have reflected a state of physiological undernutrition or an adaptation of the liver and rumen to increasing milk production. The total cholesterol was parallel with milk production from the 3rd to 13th month post partum. The lowest cholesterol values occurred in the serum of dry cows. Significant seasonal variations were noticed in PBI and cholesterol levels. The highest average PBI levels were obtained in April, May, and June and the lowest at the end of July and at the beginning of August. The cholesterol levels were highest during the indoor feeding period from December to May and lowest during the pasture feeding period. Since seasonal variations of total cholesterol level resembled greatly those of PBI, it was assumed that there are common factors influencing simultaneously both the PBI and cholesterol levels.

Effects of season and stage of lactation on protein bound iodine and total cholesterol in serum of dairy cows