Originally Posted by
jpatokal
They don't. Thais
aspirate the sound, pronouncing it with a puff of air that you hear as an "H". It's entirely different from the rather unusual
dental fricatives spelled "th" in English.
It's the aspiration that can change the consonant to what is best described by Th. In Indian alphabets, the origin of Thai alphabets unaspirated and aspirated consonants alternante such as k, kh; g,gh (westernes cannot make that sound without training), b, bh;t,th etc.