Does tea cause iron deficiency?

No: Tea or toast does not cause iron deficiency at all. But if you live off pretty much just things like tea, toast, alcohol, etc., then you will become iron deficient from lack of nutrients, including iron.

Tea & low iron: Tea doesn't cause iron deficiency, but it can block the absorption of the iron in vitamins or food if you take them together. Calcium in milk can also block the absorption of iron. Vitamin c improves iron absorption.