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Abstract
Plasma hormone changes after thyrotropin releasing hormone (TRH) injection were estimated in 65 heifers. The repeatabilities observed for basal plasma levels of thyroxine (T4), triiodothyronine (T3) and thyroid stimulating hormone (TSH) were 0.80, 0.53 and 0.61, respectively. For TRH response measured as maximum deviation from the basal values, or as area below the response curve, the repeatabilities were between 0.3 and 0.5. Correlations between production traits, and between production traits and thyroid traits were estimated. TRH response measured as the area below the response curve was positively correlated to milk yield in first lactation for both T3 and T4. For TSH, the plasma level was positively correlated to milk yield. Feed consumption in heifers was negatively correlated to yield in their first lactation.