Hi David,
I have two (now teenage!) daughters. Thalassaemia affects sperm production and reproductive growth if your pituitary glands are affected by iron deposits - so, as Andy mentioned, it is vitally important to ensure that your iron levels are under control and Serum Ferritin levels are below 1000, preferably.
Some haematologists also recommend that additional Human Growth Hormone (Testosterone) injections be taken during the puberty phase, if your growth cycle is below average or iron levels are higher than recommended. Typically, sperm production can also be improved or restored using HGH, if puberty has been successfully completed. However, keep in mind, that if you are into competitive sports, then HGH is a proscribed drug.
I had my kids when I was in my mid-20's - this was on the recommendation of my haematologist, who said it is better to have your children early, if you are planning to have them naturally. It was sound advice, in more ways than one.
Hope that this is useful for you, David, and if you have any further questions, let me know.
Cheers
Poirot