I have bought both a lab-grade (99.8% pure) Lugol's powder for making myself a long-lasting source of Lugol's iodine..
And potassium Iodide crystals, to feel the difference when using a gentler iodine.
I've used both types internally, and as skin application.
I like potassium Iodide (also known as SSKI ) for internal use, and Lugol's for topical use
GSH availability can be evaluated by laboratory tests, but it is also the big obvious..
If you are having chronic illness, have trouble maintaining a positive outlook on life, suffer chronic pains or infections, then your GSH status is lowered.