Vitamin D is regularly prescribed for thals and it has nothing to do with rickets. Vitamin D is absolutely necessary for proper absorption of calcium, which is another supplement normally prescribed for thals. The two act together to help prevent the bone loss which occurs in thalassemia.
Sunlight is an excellent source of vitamin D. Daily exposure for 10-30 minutes will normally provide all the D one needs, although the requirements of thals may be greater, resulting in the need for supplementation. Dark skinned people need more sun than fair skinned, in order to create the same amount of D. It is always advised to avoid the mid-day sun.