From what I can see from a google, low SF is a precursor of iron anemia It is desribed as an earlier and more sensitive indicator of iron deficiency than hemoglobin levels.As the iron reserves get depleted, the anemia will eventually become reflected in the hemoglobin levels. It seems that you might be having anemia symptoms at this stage even though it isn't obvious in the blood work.
There was also mention of this being associated somehow with low thyroid disease or celiac ( gluten intolerance), presumably causing malabsorption.
Treated with iron supplements although one site says should be correctable with dietary iron.
I didn't make out what the relationship with hypothyroid might be and I suppose that if it is being caused by malabsorption iron oral supplements won't help all that much. Since I have no special medical knowledge I am not sure about that conclusion.
Just thought I would mention that I am the same, I was only diagnosed with SLE a few months ago but all my blood tests have shown low ferritin levels. I tried iron tablets but I couldn't go to the loo (sorry!) but I have tried Spatone which is a sachet of iron water which you mix with orange juice and that seems to be ok. Hope this helps.