Low thyroid and/or testosterone concerns

[SchwarzeSonne]

I'm a 21 year old male and am generally fairly healthy.  However, I sometimes get fatigued easily, lack enthusiasm, have brittle/lifeless hair, suffer a little from heart palpitations and find it impossible to lose and keep off weight.  Further to this, I feel as though I am "under developed" - as if I started puberty but not quite got to the end.  All the men on my mother's and father's side are hairy, well built and somewhat musclar, while I'm weak and have practically no body or facial hair.  Furthermore, my penis hasn't developed properly and I'm still where I was when I was about 14.  

Thanks to the wonder's of the internet, I've looked into testosterone and thyroid problems.  Thyroid problems run in my family - my mother, my mother's brother and mother's mother all have under active thyroids, so the probability of me having an underactive thyroid is non-trivial.  Furthermore, according to one (reliable medical) website, under active thyroids in adolescence can lead to abnormal growth and under development during puberty as testosterone levels are abnormally low.  

So my questions:

1) In theory, if I have an underactive thyroid which was presnt during puberty, would that lead to under development, ie, under developed seuxal organs, no body hair etc...?

2) Is there another condition that I do not know about that could explain the above, eg, low testosterone levels on their own?  

3) If it is low thyroid (and/or testosterone) will I ever fully develop?  If yes, how long will it take before I notice changes?