Thyroid function in relation to trimethylamine oxidation in immature domestic fowl fed on rapeseed meal

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Abstract
Two groups of 12-wk-old female chicks were selected for their ability to oxidize 14C-trimethylamine (TMA) after being fed on a diet containing 100 g high-glucosinolate rapeseed meal (RSM)/kg for 14 days. A 3rd group (control) was fed on a rapeseed-free diet. Measurements of thyroid activity and hepatic TMA oxidase activity were made after a further week on the diets. After feeding RSM, hepatic TMA oxidase was very low (P < 0.001) in 1 group (sensitive) but was unaffected in the other group (resistant). Thyroid size was increased in both groups receiving RSM but was larger (P < 0.001) in sensitive birds. Plasma half life of thyroxine (T4) and its metabolic clearance rate were not affected by feeding RSM; thyroidal secretion was reduced (P < 0.05) to the same extent in both groups. Plasma concentration of triiodothyronine was decreased in both groups after feeding RSM (P < 0.05); plasma T4 concentration was increased (P < 0.04) only in sensitive birds. The results indicate that the short-term depression of TMA oxidation in birds sensitive to RSM is not mediated by the thyroid.