The aim of this study was to investigate the influence of the natural mineral adsorber clinoptilolite on serum levels of thyroid hormones in newborn calves in the first 48 hours postpartum. A total number of 68 newborn Holstein calves divided in four groups were used in the present study. Calves were randomly assigned to one of the following treatments: 1) 0.75 L of colostrum in 12h intervals, with 5 g/L of clinoptilolite immediately after birth (0h), at 12h and 24h after birth (0.75+); 2)

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... of colostrum in 12h intervals (0.75-), which represents the first control group; 3) 1.5L of colostrum in 12h intervals, with 5 g/L of clinoptilolite immediately after birth (0h), at 12h and 24h after birth (1.5+), and 4) 1.5 L of colostrum in 12h intervals (1.5-), which represents the second control group. The calves were born with high blood serum thyroid hormones concentrations (9.7-13.5 nmol/L for T 3 and 201-235 nmol/L for T 4). At 6. hours after birth serum thyroid hormone levels increased in all groups, but become significantly lower at 48. hours after birth. Clinoptilolite treatment could influence the rise in blood serum thyroid hormones concentration during the early postnatal period. This was most evident in the treated group of calves that received 1.5 L of colostrum (T 3 , 6h, 11.7±3.4:19.4±7.4, p<0.01; 1.5+ vs. 1.5-). Results indicate that there is a possible effect of clinoptilolite treatment on blood serum thyroid hormones concentration in newborn calves during the first 48 hours of life.