I know that calcium and iron can block absorption. As well as a high fiber diet. You can take calcium and iron supplements 4 to 6 hours from taking your Synthroid(is what I have learned).
In the morning I try not to have any dairy products for at least 4 hours from taking my thyroid medicine.
Have been taking 200mcgs of Selenium split in half. Take 100mcgs of Selenium 4 hours after thyroid medication. Then other half about 1 or 2 O'clock. Selenium helps convert T4 to T3.
Am thinking the Selenium has helped the arthritis in my shoulder.