BC Cancer Breast Screening doesn't recommend the use of thyroid guards for screening mammograms because they can interfere with capturing a clear breast image. The guard can limit the technologist’s ability to place all of the breast tissue on the compression plate. As a result some breast tissue may be missed or more x-rays may need to be taken.
Also, the screening mammography x-ray directs the radiation beam towards the breast tissue, so the thyroid isn't directly exposed to the radiation beam. The thyroid can be exposed to "background scatter radiation”, which is the same amount of radiation a North American would be exposed to from 30 minutes of natural background radiation.