Effects of Age, Body Weight and Sex on Pituitary-Thyroid Axis in Dairy Cattle and Daily Variations of Thyrotropin and Thyroid Hormones

The effects of age, body weight and sex on basal plasma levels of thyrotropin (TSH), thyroxine (T4) and tri-iodothyronine (T3) and the daily variations in these hormones were investigated in dairy cattle. The plasma levels of TSH, T4 and T3 from Holstein calves, heifers and lactating cows were measured by radioimmunoassays. The basal level of TSH showed significant positive correlations with age and body weight. On the contrary, the basal T4 level showed significant negative correlations with these parameters. Negative, but not significant correlations were observed between the basal T3 level and age and body weight, and between the basal level of TSH and those of T4 and T3, respectively. A significant positive correlation was observed between the basal level of T4 and that of T3. In the calves, no sex differences in the basal levels of TSH, T4 and T3 were observed. In addition, each basal levels of these hormones in calves, heifers and lactating cows, except the T3 in heifers, were not significant differences between May and November, respectively. TSH level in lactating cows did not show a clear daily variation, but T4 and T3 levels elevated from 4:30 (August) or 7:30 (February) to 22:30 followed by the decline. However, the amplitudes of variations of these hormoenes were small. The variation of T3 was parallel to that of T4. The results suggest as follows; 1) the basal plasma level of TSH in dairy cattle elevates during aging and increases the body weight, 2) the basal plasma T4 level in dairy cattle decreases during aging and increases the body weight, 3) plasma T4 and T3 levels in lactating cows show the daily variations having the small amplitudes.