Are growth plates likely to be fused at 18 if i had a growth hormone deficiency?

Depends on puberty: The closure of the bony growth plates is mainly regulated by pubertal changes in sex hormones (estrogen in girls; testosterone in boys). If you had only gh deficiency but with normal sex hormones, then the growth plates usually fuse around puberty. With pan-hypopituitarism, (gh and gonadal hormone deficiency), the growth plate closure will depend on how quickly and how much sex hormones are given.

Not necessarily: Were you treated? For how long? A simple x-ray can tell your doctor whether you have any other growth potential.