The goal of the study was to map the blood serum concentrations of thyroid hormones in various age categories in cattle, small ruminants, and swine. Blood serum concentrations of TSH, T4, and T3 were analysed in randomly selected healthy animals of various species and age. Age-dependent changes of thyroid hormones were studied in 7 clinically healthy calves during the first 6 months of life. The observation started at the age of one month (body weight 45 - 51 kg). A similar study was done in 9 sucking piglets since birth to 9 weeks of age (weaning at 35 days). Blood samples were taken every week. In calves, heifers, and dairy cows we recorded significant differences in T4 and T3 values with the highest levels in heifers (p<0.05). The hormones mean concentrations in sheep, goats, and swine corresponded to data reported by other researchers. Within age dependent studies, mean concentrations of TSH showed no significant changes, however, most of the samples showed concentrations below the detection limit of the assay. In the third month close to weaning we found significantly lower serum T4 concentrations (p<0.05), compared with the initial value. Similar, but however insignificant decrease was observed for T3 concentrations. In the piglets, both T4 and T3 concentrations showed a significant (p<0.001) decrease during the study with the highest mean concentrations of T3 and T4 in the youngest piglets. After weaning, both in calves and piglets concentrations of the hormones slightly increased.