Do calcium supplements interfere with the absorption of some medications?

Yes.: They can significantly lower the amount of thyroid hormone that gets absorbed. Because calcium is a positively charged ion, it can bind to any negatively charged chemical, including levothyroxine. Spacing the thyroid hormone at least 2 hours from calcium (including antacids such as tums), iron, multi-vitamins, and certain other meds can help keep thyroid blood levels stable.