My grand daughter was born in the US of a Thai mother (Dual National) and upon investigation her daughter had to have her birth registered at the Thai consulate in LA in order to obtain a Thai birth certificate. Once this was done she traveled to Thailand on a US passport and was issued a Thai passport here in Thailand with no issues. Thai consulate advised to do that as according to then issue of an initial Thai passport at a consulate or embassy is more complicated. Main point must have Thai birth certificate from embassy or consulate in country of birth is imperative,