Effect of Anabolic Steroids on Plasma Thyroid Hormones in Steers and Heifers

The effects of trenbolone acetate (TBA) alone or in combination with oestradiol (TBA + OE2) on growth rate, feed conversion efficiency (FCE) and thyroid hormone status were studied in 11 Friesian cross-bred steers and six Friesian heifers. Three steers and three heifers were implanted with TBA (300 mg), four steers were implanted with TBA (140 mg) + OE2 (20 mg) and four steers and three heifers were sham implanted as controls. There were significant improvements (P < 0·05) in growth rate and FCE in treated animals. Plasma samples were collected weekly for nine weeks after implantation and the concentrations of total thyroxine (T4) and the triiodothyronine (T3) uptake were measured and the free thyroxine index (FTI) calculated. TBA and TBA + OE2 treatment caused a significant decrease of T4 in steers and TBA alone decreased T4 in heifers. There was no effect of anabolic agents on T3 uptake, tints the calculated FTI was significantly lower than controls in steers administered TBA + OE2 and had a tendency to be reduced in steers and heifers given TBA alone. These results suggested that anabolic agents cause a hypothyroid status in cattle, and this condition may be the mechanism which results in increased growth and feed efficiency.