Abstract
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/100ml, and those in colostrum were significantly higher than those in matured milk (P<0.01), whereas rT3was not detectable in 76 samples tested.
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.
