Plasma thyroid hormone concentrations in growing beef steers implanted with estrogenic anabolic growth promotants

The effect of estrogenic anabolic compounds on plasma thyroid hormone concentrations was investigated in growing beef steers. Sixteen Hereford beef steers, weighing approximately 380 kg, were randomly placed into four groups and assigned to one of the following treatments: 36 mg implant of Zeranol, 36 mg implant of diethylstilbestrol (DES), one implant of Synovex-S and an unimplanted control. Total thyroxine (T4) and triiodothyronine (T3) concentrations were analyzed from plasma samples collected at different periods following the implantation using a solid phase radioimmunoassay. Free thyroxine concentration was estimated indirectly as free thyroxine assessment (FTA) from total T4 concentration and the binding capacity of thyroxine binding globulin (TBG-RDA). Overall mean plasma T4 concentration was higher in steers implanted with Synovex-S (23%, P less than 0.05) and DES (18%, P = 0.08) than the controls. When compared to controls, a slight increase (2.2%, P greater than 0.05) was observed in steers implanted with zeranol. The TBG-RDA values were lower (P less than 0.05) and the FTA values were higher (P less than 0.05) in steers implanted with estrogenic anabolic compounds than the controls. The FTA values were significantly increased by Synovex-S (57%, P less than 0.05) and were found to be non-significantly increased by DES (37%) and zeranol (30%) when compared to the control values. Mean plasma T3 concentration was not affected (P greater than 0.05) by DES, Synovex-S or zeranol implantation. These data indicate increased plasma total and free T4 concentration in steers due to the implantation of Synovex-S, a non-significant increase due to DES implantation and suggest that it could be one of the mechanisms by which they improve growth and metabolism. Zeranol seems to have a weaker influence on the thyroid gland than Synovex-S or DES. Plasma T3 concentration appears to be unaffected by estrogenic anabolic compounds in steers. While previous investigators have reported a suppression of thyroid gland activity in ruminants treated with androgenic anabolic compounds, the present study indicates a stimulation of thyroid gland activity, as reflected by the elevated T4 concentration, in steers treated with estrogenic anabolic compounds, especially Synovex-S and DES.