How Serious Are Carotid Artery Calcium Deposits In An Elderly Person With A History Of Thyroid Cancer And High Blood Pressure?

My first cousin had an ultra sound done of her neck because three years ago she had thyroid ca. Now she has been told her thyroid is clean but calcium deposits showed up in her carotid artery. How serious is this?
She is 81 years old and takes meds for high blood pressure.