10,000 IU units will not cause hair loss. That's why it's in the multivitamin meant for healthy hair growth. I take a multi-vitamin (not for hair, but for whole body health and have been taking it for over 12 years now) and its daily dosage of Vitamin A is:
15,000 IU Pro Vitamin A (from beta-carotene)
10,000 IU Vitamin A (from retinyl acetate)
I think toxic levels of A would be around 50,000 IU for an adult. I always feel that if the A is part of a multi and you're taking the recommended dosage, then you have nothing to worry about because I believe a lot of thought and planning goes into ensuring the combination is safe and balanced to be most efficiently absorbed. Now if you're taking a bunch of different things and each contributing some A, then you might get into those toxic levels. Otherwise if it's in a hair vitamin, then it's supposed to be like that.