Can hypnagogic hallucinations be caused by underactive thyroid? can any docs explain?

Unlikely: Hypnagogic hallucinations, which are hallucinations that occur just before falling asleep, are not a common feature of an underactive thyroid. That doesn't mean they can't be caused by by hypothyroidism, just that most people with hypothyroidism don't have them. If they began around the time someone was diagnosed, and went away with treatment, it's certainly possible.