Changes in the concentrations of thyroid hormones in dairy sheep

The aim of the research was to determine the changes in the concentrations of thyroid hormones (T3 and T4) in sheep during lactation. The experiment was conducted on 20 Polish Longwool ewes. Milking was begun after the lambs were weaned, on the 57th day of lactation. Sheep were milked twice a day (8:00 a.m. and 6:00 p.m.) until dry. The milk yield of each ewe was checked every 10 days. For determination of thyroid hormones, blood was collected every 28 days beginning at sunset, every 20 minutes for 6 hours. The average milk yield of the sheep was determined based on the individual yields on test days. The highest (P ≤ 0,01) milk yield was obtained in the first month of milking, i.e. April (0,48 ± 0,018 l). Milk yield decreased as lactation progressed. The concentration of hormone T3 was significantly higher (P ≤ 0,01) in the first and third months of lactation (0,72 ± 0,02 ng/ml in April and 0,74 ± 0,02 ng/ml in June) than in September (0,55 ± 0,01 ng/ml). The T3 concentration decreased in successive months of lactation. In contrast, the concentration of the T4 hormone increased with the progression of lactation. Significantly (P ≤ 0,01) the lowest concentration of T4 was found in the first and second month of milking (51,7 ± 9,8 ng/ml, 48,8 ± 7,4 ng/ml). The highest T4 concentration in lactating sheep was noted in August (60,55 ± 10 ng/ml). The research showed that the secretion of T3 decreases with milk yield, while secretion of T4 shows an upward trend as lactation progresses. Therefore thyroid hormones modulate secretion of milk in sheep.