The Journal of Nutrition Vol. 127 No. 2 February 1997, pp. 352-358
Copyright ©1997 by the American Society for Nutritional Sciences

Roasted Soybeans and an Estrogenic Growth Promoter Affect the Thyroid Status of Beef Steers

Manuscript received 19 March 1996. Initial reviews completed 3 May 1996. Revision accepted 7 October 1996.

Growth Biology Laboratory, Livestock and Poultry Sciences Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705-2350 and ^* Department of Animal Sciences, University of Maryland, College Park, MD 20742

We investigated the interactive effects of a roasted soybean (RSB)-supplemented diet and an estrogen ear implant [Synovex-S® (SYN), 20 mg estradiol benzoate + 200 mg progesterone] in young beef steers on measures of thyroid status before and after challenge injections of thyrotropin-releasing hormone (TRH) + growth hormone-releasing hormone (GHRH). Twenty steers (body weight 255 ± 5 kg) were assigned to the following treatments: 1) no SYN and a soybean meal-supplemented diet, 2) no SYN and a RSB-supplemented diet, 3) plus SYN and soybean meal, and 4) plus SYN and RSB. Steers were individually fed 1.13 MJ metabolizable energy/kg metabolic body wt daily of an 18% protein diet. After a 5-wk growth period, all steers were challenged (intravenous injection) over a 3-wk period with three dose levels of a combination of TRH + GHRH (0.1+0.01, 1.0+0.1 and 2.5+0.25 µg/kg body wt). There were no dose by SYN or RSB interactions. Across dose levels, values for baseline plasma thyroid-stimulating hormone (TSH) were 0.37, 0.35, 0.61 and 0.33 µg/L for treatments 1, 2, 3 and 4, respectively (SYN, P < 0.07; RSB, P < 0.01; SYN × RSB, P < 0.03; SEM 0.06). Net areas under the response curve for TSH were 66.4, 51.3, 91.4 and 64.4 (µg/L) × min, respectively (RSB, P < 0.08; SEM 12.0). Similar treatment effects and/or numerical differences after challenge were noted for thyroxine (T₄) but not triiodothyronine (T₃). Baseline (2.22 vs. 2.00 µg/L, P < 0.02) and peak (3.07 vs. 2.03 µg/L, P < 0.03) T₃ concentrations were less for steers fed RSB than for steers fed soybean meal. This study indicates that in young growing beef steers, SYN increases TSH release from the adenohypophysis and that the primary effect of RSB is reduced plasma T₃, possibly through an effect on peripheral T₄ deiodination.