Hello; I'm Not Sure If I Have An Iron/biotin Deficiency

Hello;
I'm not sure if I have an iron/biotin deficiency or if I have hypothyroidism, but my right eyebrow (only) has been thinning out quite a bit, especially near the end. I've been keeping an eye on it for a while, and it seems to go through stages where it thins out very badly and then grows back, and then the cycle continues over again. I have naturally thick eyebrows to begin with, and I don't pluck, so to see these patches where there should be eyebrow is really bothering me. I've also noticed that my eyebrows are quite brittle and dry, and tend to fall out easily. I've changed up my diet and started taking vitamin supplements to help, but it's happened enough times now where I'm wondering if there is another more serious underlying problem.
Thank you.