Omega 3 and iodine levels are also lower compared with meat eaters, as are vitamin B12 levels. Vitamin B12 is most often obtained from animal foods, and higher rates of deficiency have been found in vegans compared with other vegetarians and meat eaters. The symptoms can be serious and include extreme tiredness and weakness, poor digestion and developmental delays in young children. Untreated, vitamin B12 deficiency can cause irreversible nerve damage.
While getting less than the optimal amount of B12 is quite common in pregnant women and in less-developed countries, the reported frequencies of deficiencies among vegetarians and vegans in developed countries vary greatly in severity between age groups. Even low levels of vitamin B12, but not enough to be classed as deficient, may be bad for your health and increase your risk of heart disease.
More on Vitamin B12 here.