Age-related functional changes of total thyroid hormones and glycosaminoglycans in growing calves.

Background and Aim: During the physiological growing, thyroid and proteoglycan glycosaminoglycan (GAG) changes dynamically occur, according to genetic and non-genetic factors. The purpose of this research was to compare the effects of early postnatal development (10 days) until 210 days of life on the triiodothyronine (T₃), thyroxine (T₄), the relative T₄:T₃ ratio, and GAGs profile, and to define the different reference intervals of the calf’s development through the various growing phases. Materials and Methods: The effect of growing on total thyroid hormones and GAG profiles was studied from 10 days to 210 days of age in 64 clinically healthy Brown calves, 30 males and 34 females. Blood samples were collected at 10, 20, 30, 60, 90, 120, 150, 180, and 210 days of age. Results: The results showed a significant effect of a calf’s growth on T₃, T₄, and GAG values (p<0.0001). Significant correlations between T₃ and T₄ were observed. Compared to the previous time point, T₃ showed a significant decrease at 20 days and at 60 days (p<0.01), while a significant increase was observed at 90 days and 210 days (p<0.05); T₄ showed a significant decrease at 20 days (p<0.01), while significant increases were observed at both 180 days and 210 days (p<0.05); GAGs showed a significant increase at 120 days and 210 days (p<0.05). Positive and significant correlations between BW and GAGs in both males (p<0.0057) and females (p<0.0059) were observed. Conclusion: It can be concluded that the highest T₃ and T₄ concentrations have been associated with the early growing process (10 days), with an increasing trend also at 210 days, it is possible to hypothesize a probable metabolic effect of thyroid function in anabolic and/or catabolic directions during the calves’ development. Likewise, it can be reasonably inferred that the highest plasma GAGs at 210 days may be due to their metabolic role during the development of growing calves. Taken together, these findings suggest the potential and relative contribution made by thyroid and GAGs effects on the dynamics of growing calves.