Separation and Radioimmunoassay of T ₃ and T ₄ in Human Breast Milk

Abstract

There is little agreement among published reports of the radioimmunoassayable thyroid hormone content of breast milk, likely due to wide variations in methodology applied. In order to achieve a higher degree of specificity in the determination of T₃ and T₄ concentrations in breast milk, samples were ethanol-extracted and then chromatographed on an LH-20 column. Using this method, all T₃ and T₄ RIA activity eluted with the void volume. Following pancreatin digestion and subsequent extraction of whole milk samples, void volume T₃ RIA activity decreased, and T₃ co-eluted primarily with a standard preparation of T₃or ^I25I-T₃, at a concentration of 275 ± 132 ng/dl (mean ± SD) (n = 9). In contrast, the elution volume of T₄ RIA activity appeared unaffected by pancreatin. These data indicate that immunoreactive T₃ and T₄ are differentially bound to a thyroid hormone ‘binding’ substance present in breast milk. They further support the hypothesis that thyroid hormone sufficient to supplement the thyroid economy of the thyroid-deficient suckling infant is present in human breast milk.