Synthetic preparation of human thyroid hormone T3 used in the treatment of hypothyroidism. Since the last update, we have not found published data on its excretion in breast milk. T3 is a natural component of breast milk, found in small concentrations that may or may not depend on plasma levels (Zhang 2013, Oberkotter 1989, Jansson 1983, Mizuta 1983, Sato 1979). Its concentration in breast milk is much higher than that of levothyroxine (T4), which is usually undetectable (Jansson 1983, Sato 1979). Until there is more published data on this drug in relation to breastfeeding, safer known alternatives (levothyroxine) may be preferable, especially during the neonatal period and in case of prematurity. Associations which are experts in thyroid diseases recommend treatment with levothyroxine in breastfeeding mothers affected by hypothyroidism (Alexander 2017).