After TT you are put on thyroid hormones, the Doc then tries to get you on a level which is stable and this may take some time but does happen.
Usually a 6-12 mmonth period is needed to get the levels stabilised and then pregnancy can occur and whilst pregnant, your wife/partner is monitored closely.
A TT doesnt affect your fertility rate....its the thyroid replacement hormones afterwards that does,'
Although my personal opinion is you have a better chance of a producing a child afterwards.
But remember you must also have your male hormones checked and may need testosterone to boost the fertility rate,
I hope that helps :o)