L-thyroxine is used as a substitute in cases of hypothyroidism (at least 1 ng/kg daily for adults), and, for this reason, is not problematic. The normal thyroid content of mother's milk is approximately 1 ng/1. An infant takes in about 0.15[ig/kg in 24 hours; this represents about 1% of a substitution dosage at this age (lOpg/kg daily). This amount does not influence the thyroid function of a healthy infant. The same applies for treatment (substitution) of a maternal hypoparathyroidism.
Of course, this also means it has no therapeutic effect in case of a congenital hypo- or athyroidism. This has to be taken into account in case of extremely premature newborns with a higher risk for hypothyroidism. Neither breast milk nor formula contains enough thyroxine for substitution (van Wassenaer 2002).
Thyroid receptor antibodies (TRAb) can result in transient neonatal thyroid disease by transfer through milk from mothers treated for thyrotoxicosis, Serum TRAb concentration in neonates decrease continuously with time after birth. The calculated half-life for offspring-serum and breast-milk TRAb was calculated as approximately 3 weeks and 2 months, respectively. Transient neonatal thyroid disease may be worse and more prolonged during breastfeeding as a consequence of TRAb in breast milk (Tbrnhage 2006).
Recommendation. Substitution of thyroid and parathyroid hormones establishes a physiological state, and, thus should be continued during breastfeeding if necessary. Thyroid hormones should not be given together with thyrostatics, because higher dosages of thyrostatics would then be necessary.
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For many years, scientists have been playing out the ingredients that make breast milk the perfect food for babies. They've discovered to day over 200 close compounds to fight infection, help the immune system mature, aid in digestion, and support brain growth - nature made properties that science simply cannot copy. The important long term benefits of breast feeding include reduced risk of asthma, allergies, obesity, and some forms of childhood cancer. The more that scientists continue to learn, the better breast milk looks.