Effect of hypothyroidism on kinetics of metabolism of very-low-density lipoprotein in mares

Objective—To compare kinetics of the metabolism of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) before and after thyroidectomy in mares. Animals—5 healthy adult mares. Procedure—Thyroidectomy was performed in euthyroid mares. Kinetics of VLDL apoB metabolism were measured before and after thyroidectomy by use of a bolus IV injection of 5,5,5-2H3 (98%) leucine (5 mg/kg) and subsequent isolation of labeled amino acid from plasma and VLDL. Labeled leucine was quantified by use of gas chromatography-mass spectrometry. Production rate (PR), delay time, and fractional catabolic rate (FCR) were calculated for the 2 forms of equine VLDL, apoB-48 VLDL, and apoB-100 VLDL. Plasma lipid concentrations were measured, and VLDL composition was determined. Results—Physical appearance of horses was not altered by thyroidectomy. Significantly lower mean blood concentrations of thyroid hormones and nonesterified fatty acids were detected following thyroidectomy. Mean percentage of free cholesterol in VLDL was significantly higher after thyroidectomy. Mean plasma VLDL concentration or kinetics of apoB-48 or apoB-100 were not significantly altered by thyroidectomy. Mean ± SEM PR was significantly lower (8.70 ± 1.61 mg/kg/d) and mean delay time significantly longer (1.58 ± 0.12 hours) for apoB-48 VLDL in euthyroid mares, compared with values for thyroidectomized mares (16.15 ± 2.24 mg/kg/d and 0.93 ± 0.10 hours, respectively). Conclusions and Clinical Relevance—Hypothyroidism did not significantly alter plasma VLDL concentrations or kinetics of VLDL apoB metabolism. Metabolism of apoB-48 VLDL differed significantly from that of apoB-100 VLDL in euthyroid mares. (Am J Vet Res 2003;64:1052–1058)