Standard deviation above or below same sex / age peers who’ve been tested using the same method as the lab you’re using.
Every lab’s method deviates a little in the way IGF-1 is calculated, based on equipment and protocol. Z is the most precise measurement of where your IGF is compared to normal.
-2 to +2 physiologic range
0 - average
< -2 maybe hypopituitary / adult GH deficiency
> +2 maybe acromegaly (+3 before a concern)
Two tests less than -1, with symptoms of low GH (hard to build muscle, low energy/mood, midsection fat etc) might be a sign of “partial adult growth hormone deficiency” some leading edge endocrinologists will prescribe rHGH for.