Autoimmune thyroid disease (AITD) and type 1 diabetes mellitus (DM1) are much more common in Down Syndrome (DS) in comparison to the general population. The onset of DM1 may occur earlier than in the general population, often before age of 2 years, but AITD presents usually in the 2-3 decade of life, only sporadically before 10 years of age. The authors present a rare, unusual course of coincidence of AITD and DM1 in a 2-year-old girl with DS and congenital subclinical hypothyroidism.