Yes it is!
I am not finding information newer than the 1960s(!) pertaining to research on thyroid function and eggshell formation. The .3mgs is from a study done in the 40s (IIRC), and it was only briefly about eggshell strength.
Messing around with hormones is NOT something I'm prepared to do on my own. This was just the scratching of the surface, and that surface is buried under decades of dust.
Right now, I was reading about trace minerals, specifically Mn, Zn, and Cu (manganese, zinc, and copper), and how they can impact shell formation/strength/thickness. I'll look into it more, see what I find.