Effect of nitrogen intake on and rhythmicity of circulating thyroid hormones in steers

Abstract

This experiment was undertaken to investigate the effects of dietary nitrogen level and solubility on circulating thyroid hormones, to investigate the relationship of plasma 3-methylhistidine to changes in circulating thyroid hormones and to investigate rhythmicity of circulating thyroid hormones in steers. Isocaloric, semipurified diets were arranged factorially with three levels of nitrogen and two nitrogen sources that varied in ruminal solubility. Four rumen cannulated Hereford steers (mean ± SEM live weight 239 ± 4 kg) were assigned to each of the six treatments. Jugular blood samples were obtained every two hours for 48 hours for analysis of plasma 3,5,3′-triiodothyronine (T 3) and thyroxine (T 4). Plasma T 4 increased with increasing nitrogen level ( P<.10) but was not affected by nitrogen source (P>.10). This increase could have been related to a trend toward higher digestible energy intake with higher nitrogen level diets. Plasma T 3 was not affected by nitrogen level or source (P>.10). Plasma 3-methylhistidine:creatinine ratio (3MH/CRE) was higher in the lowest nitrogen level treatment (P<.01) compared to the other two nitrogen levels and higher in the low nitrogen solubility treatment (P<.05) compared to the high nitrogen solubility treatment but 3MH/CRE did not show a strong correlation with either plasma T 3 or T 4. Time and day of sampling effects on circulating thyroid hormones were not significant (P>.10) by analysis of variance, but trends toward periodicity corresponding to a 12-hour feeding period (T 3) and day length (T 3) and (T 4) were apparent as revealed by harmonic time series analysis.