I have heard of overdosing causing hair loss but never the other way around. Overdoing vitamin A-containing supplements or medications can trigger hair loss, according to the American Academy of Dermatology. The Daily Value for vitamin A is 5,000 International Units (IU) per day for adults and kids over age 4; supplements can contain 2,500 to 10,000 IU. But at the same time, as you said, D3 does cause hair loss when levels are not good. You do need A to absorb/make D3. I have heard of cod liver oil reversing hair loss in some women and men and that has both A and D3 in it.