IODIDE METABOLISM OF THE THYROID AND MAMMARY GLANDS DURING LACTATION IN SHEEP

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Abstract
SUMMARY: Seven ewes and nine lambs were used for a study of thyroid function and 131I metabolism during lactation. Carrier-free 131I was injected intramuscularly into the ewes, and its distribution into the thyroid, blood and milk studied in detail for 30 hr. From 30 hr. until 400 hr. less frequent blood and milk samples were taken, and thyroid count rates measured. Blood was analysed for protein-bound iodide (PBI), protein-bound and total 131I; milk for total 131I. From these data a number of parameters of thyroid function were calculated. The rate constant for thyroid 131I release, and the PBI levels, showed no difference between lactating and non-lactating ewes. Thyroid 131I uptake was reduced, due to mammary competition. Milk:serum 131I ratios varied between 29:1 and 92:1. The mammary gland was shown to be more than three times as efficient as the thyroid at removing iodide from the blood stream.