The effect of beta-carotene on thyroxine and cholesterol levels in pregnant heifers before and after parturition

Thyroxine (T4) and cholesterol concentrations and their dynamic changes and differences were studied in 20 pregnant heifers 10 days before calving until 56 days after calving. Ten control heifers were fed the feed ration for pregnant heifers according to Czechoslovak Standard CSN 46 7070. The feed ration given to another ten (experimental) heifers was fortified with 300 mg of synthetic beta-carotene per head/day within the period of five months before insemination, during pregnancy and till the 56th day after parturition. As suggested by the regression function of straight line, no obvious differences were recorded in the dynamic changes of T4 before parturition and during the first days after parturition. T4 concentrations had a generally descending tendency in both groups, the lowest values being recorded in the 48th hour in the experimental group and the 24th hour in the control. A marked increase of T4 concentrations with the values significantly higher in the control group than in the experimental group was observed between the 21st and 35th day after calving. A medium increase of cholesterol concentrations was found in the heifers given rations fortified with beta-carotene. No significant differences in cholesterol concentrations were observed between the two groups, except in the 72nd hour and the seventh day. Significant T4: cholesterol correlations were recorded in the experimental animals in the first and second hours, on the 42nd day (P less than 0.05), and in the 72nd hour (P less than 0.001); in the control group these observations were made the seventh day before calving and in the sixth hour and on the 14th, 21st and 56th days (P less than 0.05) after calving. It is inferred from the results that beta-carotene interferes with the activity of the thyroid gland and the production of its hormones, and that the increases or decreases of the activity of this gland, caused by beta-carotene, influence the metabolism of cholesterol in the body.