The use of "th" in English to represent the "th" sound originates from the transliteration of the Greek letter "theta" (θ). 
This convention was established when Greek words were adopted into Latin and subsequently into English.

Inglish uses þ.

Examples:
    theory: From Greek θεωρία (theoria), meaning "contemplation" or "speculation."
    therapy: From Greek θεραπεία (therapeia), meaning "healing" or "treatment."
    thermometer: From Greek θερμός (thermos), meaning "hot" + μέτρον (metron), meaning "measure."

Inglish:
    þiry
    þeropy
    þêmómetre