Safety in Lactation: Thyroid hormones

Milk levels of thyroid hormones have not been measured after exogenous administration in humans. However, endogenous liothyronine appears to transfer into milk in higher concentrations than levothyroxine.
Supplementation aims to bring the mother to a euthyroid state, and therefore the risk of using exogenous thyroid hormones is no different than in a normal euthyroid mother.