Turkish Journal of Veterinary & Animal Sciences




Circadian variations in serum triiodothyronine (T_3), thyroxine (T_4), free triiodothyronine (fT_3), and free thyroxine (fT_4) levels were investigated in Iranian ewes. For that purpose, blood samples were collected from the jugular veins of 15 adult clinically healthy Iranian nonpregnant uniparous ewes every 6 h (0600, 1200, 1800, and 0000) during 6 days in summer with a mean ambient temperature of 40 °C. Serum T_3, T_4, fT_3, and fT_4 were measured by radioimmunoassay. There were statistically significant differences in thyroid hormone concentrations (T_3, T_4, fT_3, and fT_4) at different hours of the day. The results revealed that serum T_3, T_4, fT_3, and fT_4 levels were highest at 1800 (P < 0.05). Significant 24 h periodicity in the serum levels of T_3, T_4, fT_4 and fT_3 was observed in nonpregnant uniparous Iranian ewes. It is concluded that circadian rhythm was observed in serum thyroid hormones.

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