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.
Sometimes. Calcium can interfere with thyroid hormone absorption and some other medications. This information is usually given with the medication.
Does high VitD3 & Calcium supplements interfere with absorption of Iron & Omega3 Fatty Acid from food for 57+ yr female?
They help each other. They do not compete directly. In fact, consuming essential omega-3 fatty acids can increase vitamin D's ability to increase calcium absorption from the gut. This is especially important in improving bone mineralization and decreasing bone loss seen in osteoporosis. Vit D3 and omega-3 together can also improve the risk profile for peripheral vascular disease.
Should I carry on taking calcium supplements whilst having iron injections / infusions? Will it affect the absorption of the iron?
You would be. Better off having 5 dairy servings a day in your diet than taking calcium supplements. It is best absorbed that way. No oral intake will interfere with anything put directly into your vein. That is why they give the infusion do there is no worry about absorption.