Carnivores in general tend to have high levels of vitamin A in their livers, polar bears simply more so. Yes, it probably is connected to their diets.
Polar bears are more exclusively carnivorous than any other bear species and are the sole land predator that lives off fatty marine mammals with loads of vitamin A, which concentrates in the top predator’s liver. But even polar bears are safe to eat, apart from the liver.
Once again, this powder contains no significant Iron.
If I may be pedantic for a moment . . .
Excessive vitamin D intake doesn’t screw up your parathyroid glands so much as it raises the blood calcium level (a potentially lethal derangement, although much more likely to just make you feel like shit).
In other words, everything else being equal, if excessive vitamin D intake has led to high blood calcium levels, the parathyroid glands will simply turn off* - not something that’s bad for them in any way. They should kick back in once the blood calcium levels drop back into the normal range (and/or lower than normal).
(*since the primary function of the four parathyroid glands is to raise blood calcium levels, i.e. a nice example of part of a homeostatic feedback mechanism)
Note what the title of the thread is.