Presence of Triiodothyronine, No Detectable Thyroxine and Reverse Triiodothyronine in Human Milk

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Thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) concentrations in human milk were measured by radioimmunoassay in 114 samples obtained. from 1 week to 8 months postpartum. Several assay systems applied for the determination of serum thyroid hormone concentration were proved to be unsuitable for human milk, and the method of separating free and antibody-bound hormone by polyethylene glycol was also inappropriate for milk specimens, which tended to give a falsely high value. The binding affinity of T4to milk was lower than that to serum protein, on which 8-anilino-1-naphthalene sulfonic acid showed no remarkable effect. In spite of the high sensitivity of 100pg/tube in T4 assay system, no immunoassayable T4 was detected in all samples with or without ethanol extraction and trypsin hydrolysates of milk. In contrast, T3 was present in a measurable amount in most of the samples, the mean±SD value of which was 10±9ng/100m<I>l</I>, and those in colostrum were significantly higher than those in matured milk (P<0.01), whereas rT3was not detectable in 76 samples tested.<BR>These results indicate that permeability of thyroid hormones through the mammary gland is different between T4 and T3 as well as in placental transport, and human milk can not be a source of thyroxine supply for the breast-fed infant.
社団法人日本内分泌学会の論文