Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2001
Publication Date: November 1, 2001
Citation: Capuco, A.V., Wood, D.L., Elsasser, T.H., Kahl, S., Lefcourt, A.M., Piperova, L.S. 2001. Effect of somatotropin on thyroid hormones and cytokines in lactating dairycows during ad libitum and restricted intake. Journal of Dairy Science. 84(11):2430-2439.
Interpretive Summary: It has been postulated that the effects of growth hormone on milk production are mediated by thyroid hormones. This being accomplished by virtue of growth hormone inducing changes in thyroid hormone metabolism at the tissue level. Although other data support a role for thyroid hormones as mediators of increased milk production in rats and mice, data from this experiment do not support a role in dairy cows. Cows were treated with bovine growth hormone when feed intake was limited, to mimic the condition during peak lactation when cows utilize body reserves to obtain sufficient energy to support high levels of milk synthesis., and when feed intake was not limited to mimic conditions during the majority of a cow's lactation. In nether case were there changes in thyroid hormone concentrations in serum, or metabolism in tissue that support this hypothesis.
Twelve Holsteins in first lactation were used to investigate the relationship between energy intake and effects of bovine somatotropin (bST) on thyroid hormone metabolism and cytokine concentrations in serum. Cows averaged 132 ñ 8 d in milk at the beginning of the study. Six cows were fed for ad libitum intake and 6 cows were fed for 80% of ad libitum intake to induce negative energy balance during two treatment periods of 7 d. During treatment periods, cows were administered vehicle or 40 mg bST/d according to crossover design. Between treatment periods there was a 14-d recovery period, during which all cows were fed ad libitum. Milk yield was reduced 10% by feed restriction and was increased by bST treatment in both dietary groups. Plasma NEFA were increased by diet restriction and by bST. Serum concentrations of IGF-1 did not differ between dietary groups but were greater during bST than control (87 ng/ml) periods. Serum thyroid hormone concentrations were reduced by dietary restriction but there was n effect of bST treatment. Total thyroxine (T4) averaged 58 and 48 ng/ml, triiodothyronine (T3) averaged 2.7 and 2.4 ng/ml and reverse T3 averaged 0.16 and 0.11 ng/ml, for ad libitum and restricted feeding, respectively. Ratios of serum T3/T4 and reverse-T3/T4 were not influenced by diet or bST treatment. Thyroxine-5'deiodinase (5'D) the enzyme that converts thyroxine to the active thyroid hormone, T3, was not affected by restricted intake or by bST treatment. Plasma concentration of tumor necrosis factor, a potential regulator of 5'D, was not affected by dietary restriction or bST treatment. Short-term treatment with bST did not influence thyroid hormone metabolism in lactating heifers in positive or negative energy balance.