Before I was diagnosed hypo I noticed my pubic hair getting really sparse and I thought it was maybe because I had been through the menopause and part of the ageing process and commented to my daughter at least there is one good thing to getting old as I have always hated that particular part of body hair. Since been on Levothyroxine it has thickend out again much to my annoyance so I would say yes it is caused by hypothyroidism.
I do not know about the underarm hair as I always shave there I cannot say I have noticed any difference.
I know you say your test always come back as normal but what I have learned from this excellent site is that you must get the results from your G.P with the ranges then post them on here and some of the more knowledgeable will advise you. Doctors often say they are normal when in fact you could be hypo especially with your symptoms and your Mother been hypo as well. Hypo can also make PMT worse.
Hope this helps a little.
Best Wishes browny