how far along are you? that seems like great doubling time to me! i just went back for my 3rd set of labs (i am around 6w / 27dpo) & am now at 23,700 from 2,000 a week ago, so I think my doubling time is around 48hrs...at least my doc finally okayed me to go for my first u/s on Monday =)
since hcg doubles every 24-48 hours, you are within normal. Technically, they could be even higher if yours happen to double every 24 hours. Looks like your are double maybe every 36 hours or so...
ex:
day 1- 612
day 2- 1200
day 3- 2400
day 4- 4800
day 5- 9600
day 6- 19200
etc....