The reaction of the thyroid gland to the thyrotropic hormone “load” in high and low-yielding cows

The studies were carried out on cows of Holstein black-and-white breed. 2 groups of 10 heads each were formed. The milk yield for lactation was 10 thousand kg in the first group, and - 5 thousand kg in the second group. Blood was collected from the subcaudal vein monthly during lactation. T3 and T4 were determined in the blood serum. The functional “load” on the thyroid gland was performed at the peak of lactation (3 months) and at its end - at 9 months. The TSH preparation at a dose of 0.5 u/kg of live weight was administered intramuscularly on an empty stomach before morning feeding. T3 and T4 hormones in the blood were determined before the injection of TSH and 0.5 one and two hours after the injection. Kath was calculated using the formula Т1 – Т0 / Т0 where Kath is the coefficient of activity of thyroid hormones. T0 – T3 and T4 levels before TSH injection T1 – T3 and T4 levels 2 hours after TSH injection. During lactation, the level of milk production and thyroid hormones varies inversely with the average daily milk yield. During the period of high milk productivity (2,3,4 months of lactation), the concentration of T3 and T4 in both groups was low, and later increased and was maximum in the 8th month of lactation. The functional reserves of the thyroid gland in high-yielding cows in 3 and 9 months of lactation are lower in relation to low-yielding cows