I lost a lot of my hair due to Graves disease, and have thin, splitting nails due to medication & RA. A wide-spectrum multivitamin including biotin has helped. Not a miracle cure, but I can tell the difference.
Sue

@dianechaz49
I'm still on an AI (three years out from chemo) and, yes, the biotin and fish oil have kept my new nails and hair from splitting which at my age (73) has been helpful, and they both grow faster than prior to chemo. Because of the AI, I still don't have hair elsewhere (legs and armpits) nor on two spots on my head where the chemo had caused sores. The hair at the back of my head has come in more thickly (although individual strands remain baby thin everywhere). I also try to remember that age is playing a part in all this, it's just that one cannot tell what is attributable to what. This past year, I reduced the amount of biotin from 50Kmcg in years one and two to 30Kmcg in year three. My new hair came in different colors from my old hair... white, gray, and dark brown at the back of my head, and I have not colored it nor had a permanent as it doesn't appear strong enough to withstand chemical treatments.

I used Mill Creek biotin shampoo and it literally thickened my fine hair. You can buy it on Amazon or GNC used to carry it.