Estimation of hormonal and non-hormonal iodine uptake from maternal milk in suckling rats

Abstract

Abstract. Maternal milk may carry hormonal iodine and thus contribute to a high thyroxine secretion level in suckling rats. In order to evaluate this possibility, milk iodo-compounds were estimated in rats equilibrated with 125I. On day-10 post partum, the milk contained 195±65 ng/ml iodine composed of 43 ng/ml iodide, 148 ng/ml non-hormonal organic iodine and only 3 ng/ml hormonal iodine. The stomachs of the sucklings contained 0.70 ± 0.05 and 1.2 ± 0.3 μg iodine/organ on days 10 and 20, respectively. The quantity of hormonal iodine was negligible compared with the amount of iodide and non-hormonal organic iodine. After enzymic proteolysis of the milk and stomach contents of the sucklings, 80–90 per cent of the non-hormonal organic iodine found was monoiodoryrosine (MIT), while the rest was diiodotyrosine (DIT). Sixteen hours after injecting [131I]iodide and [125I]thyroxine into the mothers not equilibrated with 125I, on day-10 0.87 ± 0.10 per cent dose/ml of [125I]T4 was found in maternal plasma, 0.2–0.4 per cent dose/ml in milk, while only 0.05 ± 0.01 per cent dose/organ and 0.008 ± 0.002 per cent dose/ml was found in the stomachs of the sucklings and in plasma, respectively. By contrast, 131I rapidly disappeared from the maternal plasma and appeared in relatively high levels in the serum of the young. These results indicate that milk thyroxine is partially destroyed in the gut of the suckling. Maternal milk is a source of iodide and non-hormonal protein bound iodine in sucklings and does not contain any hormonal iodine.