Levothyroxine, used to treat an underactive thyroid, is sensitive to what else is in your stomach. Calcium and iron supplements, antacids containing aluminium or magnesium, and some other products can bind to it and reduce how much your body absorbs, leaving your thyroid undertreated.
It is usually taken once daily on an empty stomach, often first thing in the morning, with these interacting products taken several hours apart. Certain medicines, such as some used for cholesterol or stomach acid, and even large amounts of soy or fibre, can also affect absorption.
Keep a consistent routine and tell your doctor about new medicines or supplements, as your dose may need monitoring. Contact your doctor if symptoms of an underactive or overactive thyroid return, and have your levels checked as advised.