Iodinated compounds and thyroxine binding to albumin in human breast milk

Mature human milk samples from young healthy women on an equilibrated diet contained a mean of 81 ng/ml total iodine. Iodide represented a mean of 77% of the total iodine. Of the 22 ng/ml organic iodine, there was about 1 ng thyroxine and triiodothyronine and, after pepsin hydrolysis, up to 40% of organic iodine in monoiodotyrosine form. By electrophoresis, after incubation with radioactive thyroid hormone, we found an absence of binding on thyroxine-binding globulin, but thyroxine and triiodothyronine were bound to albumin with a maximal capacity 50 times higher than in human serum. All these differences did not favor transport of iodinated compounds from maternal serum to milk. In conclusion, milk iodide was taken up by newborn thyroid to make thyroid hormones.