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Iodine, as an essential trace element, is an important constituent of an organism. A deviation from an adequate supply can lead to a loss of performance and health problems. The aim of this study was to provide a survey of the metabolism of iodine and to give recommendations for the evaluation of the iodine status of dairy cows.
The study involves two sets of data, collected by the department of herd-health of the Clinic for Ruminants and Swine of the Free University of Berlin. The first data set contains information of samples collected between 1995 and 2010 on 446 different farms. These farms are located in East Germany and keep 581 dairy cows on average, mainly Holstein-Friesians. For the analysis one pool sample was gained out of ten samples of healthy, multiparous cows, grouped by their lactation status. These 1980 pool samples were, inter alia, analysed in reference to their thyroxine concentrations.
The seconcd data set contains information of 18 farms, which were sampled between 2008 and 2010. All farms keep Holstein-Friesian cows and their farm sizes vary between 400 and 1500 cows. On every farm 21 healthy cows from three groups with different lactation statuses were chosen for sampling. The iodine concentration of each sample of serum and urine was measured individually. The samples of heparin-plasma, EDTA-plasma and serum were grouped by their lactation status pooled before analysing the iodine concentration.
The thyroxine concentration shows a significant difference according to season and stage of lactation. The concentration increases in the cold season to satisfy the metabolic needs. The concentration also rises in the course of lactation till the next birth. Post partum it significantly decreases, due to the adaptation to metabolic changes and the negative balance of energy.
The farms sampled show a very high proficiency level. The concentration of thyroxine correlates positively with the milk yield. In this study no relevant relation between the thyroxine concentration and the frequent occurrence of diseases like ovarian cysts and retained placentas could be found, neither could a correlation with reproductive measures, like the insemination index, the calving interval, the dwell time and the age of first birth be stated.
Thyroxine concentration correlates positively with the concentration of iron and erythrocytes in blood. Iron is a component of the haemprotein thyroperoxidase and thereby essential for the biosynthesis of thyroid hormones. Erythrocytes bind thyroid hormones, especially triiodothyronine, and function as a transporter. No relevant relation to other parameters of this study could be found.
Thyroxine concentration is an adequate parameter to determine the thyroid function of individuals and herds. Control mechanisms make it gradually respond to changes in supply of iodine.
The determination of the iodine concentration is also a good parameter to evaluate the iodine status of a herd. In blood and urine the concentration adapts itself quickly to the actual supply.
Plasma as well as serum samples are appropriate to determine the iodine concentration. In this study the iodine concentrations in heparin-plasma (110 μg/l) and serum (112 μg/l) barely differ.
The iodine concentration in EDTA-plasma (104 μg/l) however is clearly lower, for which no explination in the literature can be found. For this reason heparin-plasma and serum samples are to be preferred to EDTA-plasma. Urine is also an adequate parameter to determine the iodine concentration of a herd, less of individuals. There are some factors to consider, like the excessive intake of salt or goitrogens, which can influence the iodine concentration in urine.
From the results of this study reference values were calculated for the thyroxine concentration and the iodine concentration in blood as well as in urine. Considering reference values in the literature, recommendations were given. In this study values in urine vary widely, therefore the outcome is a wide reference range.