My mother have bilateral heterogeneous thyroid with micro nodules size upto 3.5mm is it very serious? And is it cancer? She takes strong multivitamin

Common. Small nodules of the thyroid are common in the population. Most of these nodules are benign. We typically do not attempt biopsy of any nodule unless it is more than 10mm in size. Nodules of 3.5mm or less are usually of no clinical significance. Taking vitamins should not affect this.