Concentration Of Thyroid Hormones In Blood Serum And Miik Of Holstein Cows At Different Stages Of Lactation And Pregnancy

The objective of this study was to investigate the effect of physiological status on the thyroid hormones concentration in both blood serum and milk of Holstein cows. Twenty eight lactating cows were divided into 4 groups, each of 7 animals; lactating nonpregnant and lactating pregnant cows; early, mid and late pregnancy. The 5th group was 7 dry cows. Blood simples were obtained from all animals, along with milk samples from lactating groups. Serum thyroxine (T4), triiodothyronine ( T3 ) and reverse triiodothyronine (rT3) and milk T4 and T3 concentration were measured by radioimmunoassay ( RIA ). Thyroid hormones concentrations of serum and milk were analyzed using Fishers least significant difference procedure. Lactating non-pregnant cows had the lowest value of serum T4 concentration, while these in early pregnancy had the highest value. Similarly, serum T3 and rT3 levels tended to be lower in lactating non-pregnant cows than those of pregnant, lactating or dry cows. Lactating non-pregnant cows had significantly the highest values of total milk T4 and T3, this may be related to high milk yield in this period. It was concluded that physiological status had a significant effect on blood serum and milk levels of thyroid hormones