This is the first case report of a 45, X male with autoimmune thyroid disease. Chromosome analysis revealed that all cells were 45, X without chromosome Y. The entire short arm and the centromere of Y chromosome were translocated onto the short arm of chromosome 18. Fluorescence in situ hybridization revealed that the SRY gene was present in all cells. At the age of 3 years he was diagnosed with hypothyroidism, and treatment was initiated.

In the rare event of 45, X karyotype in a male, hypothyroidism should be considered so that early treatment could be initiated.