Comparison of growth hormone-releasing factor and somatotropin: thyroid status of lactating, primiparous cows.

Effects of recombinant bovine growth hormone-releasing factor and recombinant bST on thyroxine 5'-deiodinase activities of liver and mammary gland were studied for lactating cows. Beginning on d 118 of lactation, 30 primiparous Holsteins received continuous infusions of 12 mg/d of releasing factor or 29 mg/d of bST or served as uninfused controls for 63 d. Blood samples were collected on d 1, 29, and 57, and samples of whole milk were collected at the early afternoon milking on d -5, 30, and 58. Liver and mammary tissue samples were obtained at slaughter on d 63. Infusion of releasing factor and bST decreased serum concentration of triiodothyronine by 10% and the ratio of triiodothyronine to thyroxine in serum by 20%. Concentrations of circulating triiodothyronine reflected a 30% decrease in hepatic 5'-deiodinase activity in response to infusion with releasing factor and bST. In contrast, treatment with releasing factor and bST did not affect 5'-deiodinase activity in the mammary gland and did not alter triiodothyronine concentration in milk. Data suggest that releasing factor and bST increase the hypothyroid status of the lactating cow and maintain an euthyroid condition in the mammary gland, thus enhancing the metabolic priority of the mammary gland.