This is just a guess but I think 10,000iu will put your level between 60-70 and 8000iu probably 50-60ng/ml. So it’s your call.
If you weigh more than 150lbs then you would need more than 5000iu. If you gained any weight, that may also be why your level decreased. But if it’s not a factor of weight, then do you have any illnesses or unusual amounts of stress? That could do it also.