pumpkin head
ËÑÇ¿Ñ¡·Í§ hua fak-tong

ËÑÇ (hua) - head
¿Ñ¡·Í§ (fak-tong) - pumpkin

The term is a literal translation.

If your "pumpkinhead" means sth like "you silly gal", I'd translate it as
"ÂѺêͧ" (yai bong <-- that's for a girl/woman) "µÒºêͧ" (Ta-bong <-- for a guy) or "µÔ꧵êͧ" (Ting-Tong).
Both words are slangs of affection, toned towards sth cute.
BUT if your "pumpkinhead" means dumb,stupid or sth in a negative way, I'd translate it as "§Õèà§èÒ" (ngee-ngao). This word is also a slang.

HTH