Hi Polly,
Have you had your vitamin D level checked? If not, ask your doctor about having it tested. It is the most common deficiency and low levels will cause your immune system to be weaker and you will also suffer more from fatigue. I am not a thal carrier and I take a dose of 50,000 IU vitamin D once weekly and it has helped me quite a bit. I started this after talking to one of the top thal doctors around and he told me that they had little success raising the D level of thals until they went to the high weekly dose, so I decided I would try it after years of unsuccessfully raising my dose high enough with lower daily doses.
I would also recommend that you take zinc daily for your immune system and that you try a sublingual vitamin B-12 tablet daily, to see if it picks you up at all. I would recommend 2500 mcg B-12 daily.