Changes of thyroid hormones concentration in blood of sheep depending on age and reproductive status

The influence of the reproductive status was tested in the blood serum of 30 sheep (10 pregnant sheep on the 15th day prior to lambing, 10 lactating sheep on the 20th day of the lactation and 10 non-pregnant sheep). The influence of age was determined in the blood serum of 20 sheep and 20 lambs. Analysis of the impact of age in sheep and lamb on the concentration of thyroid hormones showed that 30-day old lambs had significantly higher (P<0.01) concentration of T3 and T4 compared to the other age categories of sheep. At the same time, 100-day old lambs had significantly higher concentration of T4 if compared to the 1 and 3-year old sheep. Referring to the reproductive status, there was significantly lower (P<0.01) concentration of T3 determined in the blood of lactating sheep than of non-pregnant and pregnant sheep. These findings point out unsatisfactory energy supply in the older sheep, sheep during late pregnancy and sheep at the beginning of lactation.