Variations of Plasma Thyroid Hormones Concentrations and Their Percentages During Different Ages of Sarabi Calves

The thyroid is a main endocrine gland that secretes its hormones such as thyroxine (T4) and free-triiodothyronine (T3). Many factors (i.e., gender, age and other physiological status) have complex effect on their levels in blood of animals. But, there is a little information about the changes of plasma thyroid hormones concentrations during different ages of domestic animals. In this study, variations in the plasma T4, T 3, free thyroxine (f^), free triiodothyronine (f^) concentrations and their percentages, T4^ and fT4 . fT3 ratio during different ages and of different sexes in Iranian Sarabi calves were investigated. Blood samples were taken from the jugular vein of 42 clinically healthy animals in three age groups (1-14 days, 1-2 and 3-6 months) in autumn. The plasma was analyzed to determine thyroid hormones (i.e. T4, T3, fT4 and fT3) concentrations using chemiluminescence immunoassay (CLIA) method. Results indicated a significant difference in the T4 and fT4 concentrations among age groups (P<0.05) and the highest levels of thyroid hormone were seen during the first two weeks after birth. No gender-dependent significant differences and correlation in the studied parameters were found. It was concluded that age (significantly), season and gender and their related physiological and endocrinological changes have the highest effect on the plasma thyroid hormones concentrations in Sarabi calves.