Serum thyroid hormone concentrations during growth and puberty in Carora dairy heifers of Venezuela.

The aim of this study was to characterize changes in serum concentrations of triiodothyronine (T(3)) and thyroxine (T(4)) in relation to growth and the onset of puberty in Carora heifers, a Venezuelan dairy breed. Heifers aged 7 to 16 months were grouped retrospectively according to puberty status into 4 groups: Pubertal (control group; n = 12) and nonpubertal (n = 8) contemporary (born during the same week) heifers, pubertal (n = 7.) and nonpubertal (n = 7) noncontemporary (born in different months) heifers. A split-plot model with repeated measures over Months 7 to 16 of age was used. Control heifers attained puberty at 290 +/- 5 kg body weight (BW) between 13 and 14 months of age. Significant (P < 0.01) interactions of birth time of year and puberty status were detected in BW and serum concentrations of T(3), but only interaction of birth time of year was found in T(4). A transient but significant (P < 0.05) decrease in T(3) secretion was seen in pubertal contemporary and noncontemporary (101 +/- 4 and 113 +/- 4 ng/d1, respectively) heifers at Month 12 of age, a change which could be critical to the onset of puberty. Significant (P < 0.05) positive correlation was found between BW and T(3). In summary, thyroid hormone secretion changed across the months of growing and around the onset of puberty in Carora heifers.