Thyroid metabolism in sheep during pregnancy

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Abstract
1. The peripheral metabolism of thyroxine in sheep in pregnancy was investigated by measuring the rate of disappearance from blood of injected131I-thyroxine. Only limited changes in thyroid metabolism during pregnancy were observed. The rate of turnover of thyroxine slightly increased, but this was not accompanied by increased levels of plasma protein-bound iodine (PBI), or thyroxinebinding protein.2. In a 3·5-month foetus, the thyroid was active, and a marked concentration of iodide by the foetus was observed. The significance of this activity could not be assessed in the absence of knowledge on the placental passage of iodide or thyroxine in the sheep.3. Knowledge of thyroid metabolism is inadequate at present to explain any correlation between animal productivity and either hormone supplementation, or PBI concentration.