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

1. Two groups of 12-week-old female chicks were selected for their ability to oxidise 14C-trimethylamine (TMA) after being fed on a diet containing 100 g high-glucosinolate rapeseed meal (RSM)/kg for 14 d. A third group (control) was fed on a rapeseed-free diet. Measurements of thyroid activity and hepatic TMA activity were made after a further week on the diets. 2. After feeding RSM, hepatic TMA oxidase oxidase was very low (P less than 0.001) in one group (sensitive) but was unaffected in the other group (resistant). Thyroid size was increased in both groups receiving RSM but was larger (P less than 0.001) in sensitive birds. 3. Plasma half life of thyroxine (T4) and its metabolite clearance rate were not affected by feeding RSM; thyroidal secretion was reduced (P less than 0.05) to the same extent in both groups. 4. Plasma concentration of triiodothyronine was decreased in both groups after feeding RSM (P less than 0.05); plasma T4 concentration was increased (P less than 0.05) only in sensitive birds. 5. The results indicate that the short-term depression of TMA oxidation in birds sensitive to RSM is not mediated by the thyroid.