Thyroid Hormone Response to Thyrotropin Releasing Hormone by Pigs Fed Canola, Rapeseed or Soybean Meals

Pigs weighing approximately 13 kg were fed diets in which the protein supplement was either 12% soybean meal (SBM), 15% canola meal (CM), or 15% rapeseed meal (RSM). The CM and RSM originated from the cultivars Tower and Midas, respectively. After 46–59 days on the experimental diets, blood samples were collected via jugular catheters at −10, 0 and for 70 min following the infusion of 40 μg thyrotropin-releasing hormone (TRH). The preinfusion level of plasma thyroxine (T4) was depressed (P < 0.01) in the RSM-fed pigs when compared to the SBM- and CM-fed animals (1.8 vs. 3.7 and 3.6 μg/dL, respectively). Following TRH infusion, circulating triiodothyronine increased for each diet (P < 0.05) with no difference among diet groups. Plasma T4 concentrations did not increase following TRH infusion in pigs fed CM or RSM whereas SBM-fed pigs increased the circulating T4 from 3.7 to 4.9 μg/dL (P < 0.001), indicating that CM- or RSM-fed pigs had no detectable ability to increase the output of T4 in response to an external TRH stimulus. It is suggested that further studies on the use of CM in swine diets are required under conditions which require increased thyroid hormone secretion.