Underarm hair is thinning? I am not menopausal?

Okay so lately ive noticed that my underarm hair is not growing as fast as it used to and is growing in thinner and more sparse after I shave. I also noticed it with my pubic hair because I shave there too. I am only 20 yrs old so it's obviously not menopause. I read it could be a thyroid problem and last year my bloodwork showed my thyroid levels were off so they had to retest it but then they said I was fine. I don't know why else this would be happening, my leg hair grows fine and my hair on my head is as thick as it always was and i'm not losing it or anything. Anyone know what might be causing this? Has it happened to anyone else?