I think I may I have started puberty late, I'm now 17.5 of age and still lacking in some areas, like only near the end of my age 15 did my voice break and I have only recently started to grow face hair. I took the Tanner test and it told me I was at stage 3.1 (Quite a shock.) What does this mean for me? Will I still grow? I have a 5.5 inch penis and I am 186 cm tall (6'1"), and at 115 kg (253 lbs). I am on a diet and have lost 40 kg (88 lbs). Also I have an under active thyroid. Does this contribute in any way?
The thyroid is a part of the series of glands that control sexual development. Hypothyroidism (under active thyroid) can result in low levels of testosterone in your body, which in turn will slow down your development. I assume you are under a doctor's care and the doctor is monitoring your situation.
I'm assuming that either there are some aspects of your development that have not progressed steadily or, more likely, the questions were not answered accurately (based on the fact that your voice broke two years ago, you are tall, and have some facial hair). Those are characteristics of stage 4 and not stage 3. However, since I don't know you or see you, I can't say what put your result so low or what would have been better answers.
Congratulations on your diet. That is an impressive start.