Presence of triiodothyronine, no detectable thyroxine and reverse triiodothyronine in human milk.

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Abstract
Thyroxine (T4), triiodothyronine (T3) and reverse [r] T3 concentrations in human milk were measured by radioimmunoassay in 114 samples obtained from 1 wk to 8 mo. postpartum. Several assay systems applied for the determination of serum thyroid hormone concentration were 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 T4 to 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 100 pg/tube in T4 assay system, no immunoassayable T4 was detected in all samples with or without ethanol extraction and trypsin hydrolysates of milk. T3 was present in a measurable amount in most of the samples, the mean .+-. SD value of which was 10 .+-. 9 ng/100 ml, and those in colostrum were significantly higher than those in matured milk (P < 0.01), whereas rT3 was not detectable in 76 samples tested. Permeability of thyroid hormones through the mammary gland is apparently different between T4 and T3 and in placental transport. Human milk evidently cannot be a source of T4 supply for the breast-fed infant.

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