SIMPLE AND PARTIAL CORRELATIONS BETWEEN THE EPITHELIAL TISSUE CONTENT OF THE THYROID, THE BODY WEIGHT, AGE, AND LEVEL OF MILK YIELD IN AYRSHIRE COWS Vappu Kossila with technical assistance from Kaarina Heliander Department ofAnimal Husbandry, University of Helsinki Received March 19, 1969 A reciprocal relationship occurs between the level ofthyroxine in blood and the intensity of TSH secretion from the anterior pituitary. If the synthesis and release of thyroxine within the thyroid gland is in some manner (as a result of iodine deficiency, goitrogens, enzyme defects) inhibited or retarded, the blood thyroxine level will eventually decrease and, as a consequence, the TSH release rate from the pituitary will increase above normal. This, in turn, results in hypertrophy and hyperplasia of the thyroid. Under these conditions a hyperplastic thyroid may prove to be an indicator of an inadequate rather than an increased thyroxine secretion rate. It is known that thyroxine is essential in maintaining efficient milk secretion (Turner 1968). Furthermore, it is quite possible that a greatly increased TSH secretion may interfere with the synthesis and release of other anterior pituitary hormones needed for maintaining an optimal secretion of milk. Swett et ai. (1955, p. 31) observed significant positive correlation between the relative thyroid weights and the milk and butterfat yields in Jerseysbut not in Holsteins and Guern- seys. In the study of Kossila (1967, p. 73), the thyroid weight of the cows was positively and significantly correlated with the body weight and age, while it was negatively and nonsignificantly correlated with the milk yield, when the body weight and age were held constant. However, compared to the weight of the thyroid, the epithelial tissue content of the gland is probably a more reliable indicator of the thyroxine secretion rate or of the intensity of the TSH stimulation on the thyroid gland (Tala 1952, Uotila & Kannas 1952, Lamberg 1953). The purpose of this study was to find out whether there were any significant correlations between the absolute amount of epithelial tissue contained in the thyroid gland of the cows and their body weight, age, and capacity of milk production. https://www.c-info.fi/en/info/?token=kaE8X9G_KYV7DfQ7.BGGN8see22lpr-rgx5xC_g.M0HeuGdtVM5_VH88kEAL0cV5iBZgiSHN69Nz6NxRVF-2lkU2OB6msmRkPvVgUV61RVGnJ6ksY99ma5saK5FKSxofA248TeLLJmP-sQRkrZ9ebcMl_SzjF8S5Nfc8KUz2UMq43ulGXBc9ixiqr-adda7kDf0v3Gh5hxmOl7AmeCHHaZ26F1MWwj2PsTGnxw 155 Materialand methods The thyroid material was obtained from 141 Ayrshire cows discarded from the dairy herd of the Viik Experimental Farm during the years 1958—59 (group I) and 1960—65 (group II). It should be pointed out that some of the cows had received hardly any or no supplemental iodine in their diet (group I) and some had been given supplemental iodine in their diet periods ranging from some months to several years (group II) during their productive lifetime. Feeding and management of the herd, data concerning the cows, and the methods used in estimating the live weight and corrected body weight, the average daily fat-corrected milk yield (FCM) during the productive lifetime of the cows, etc., have been explained in detail earlier (Kossila 1967, p. 32—40). After first estimating the percentage of epithelial tissue in the thyroid (E %) by the method of Uotila & Kannas (1952), the absolute amount of epithelial tissue in grams contained per gland (Eg) was calculated by using E % and the fresh weight of the thyroid gland. The coefficients of simple and partial correlations were calculated according to Croxton & Cowden (1955). Before calculating the coefficients of simple and partial correlations, the data obtained from 141 cows were grouped indentically with those of the previous study (Kossila 1967, p. 71—73). The Eg (X 4) was considered as a dependent variable, and the live weight in kg (X 2), or the corrected body weight in kg (X2a ), the age in months (X 3), and theaverage daily FCM yield during the productive lifetime of the cow in kg (X 4), as independent variables. At first, the coefficients of the simple and partial correlations were calculated separately for the growing cows (N = 72), for the full-grown cows (N = 69), and for all cows (N = 141) belonging to groups I -f- 11. These results are presented in Table IA. Subsequently, the respective correlations were calculated for the growing (N = 51), for the full-grown (N = 55) and for all cows (N = 106) belonging to group 11. The results are presented in Table 18. Since group I consisted of only 35 cows (21 growing and 14 full-grown), it seemed pointless to calculate similar correlations separately for such a small group. It was thought that the effect of the cow material belonging to group I could become apparent in the results obtained from the entire material, i.e. from those obtained for groups I + II (Table 1A). Results Effect of body weight. Results in Table 1 show that the body size was postitively correlated with theEg, and that the coefficients of the simple and partial correla- tions in question were more significant between the corrected body weight and the Eg (ri2a etc-) than between the live weight and the Eg (rl2 etc.), and also more significant in groups I -j- II (especially in growing cows) than in group 11. Effect of age. All of the coefficients of simple and partial correlations between the Eg and the age of the cow (rl3 etc.) were nonsignificant, indicating that age had little effect on the Eg. Effect of milk yield. In groups I -J- 11 the coefficients of simple and partial correlations obtained between the milk yield and the Eg (r l4 etc.) were negative. They 156 Table 1. Coefficients of simple and partial correlation between the absolute amount of epithelial tissue in grams per thyroid (X,), the live weight in kg (X2 ), or the corrected body weight in kg (X 2