Answers from Todd B. Nippoldt, M.D.
Yes. Calcium supplements — or antacids containing calcium — can interfere with the absorption of levothyroxine (Synthroid, Unithroid, others), a synthetic form of thyroid hormone.
This interference happens chiefly if you take levothyroxine and calcium supplements at or near the same time. You can avoid this problem with the following steps:
Other supplements — especially those containing iron — also can interfere with absorption of levothyroxine, as can certain foods and medications. If your doctor prescribes levothyroxine, be sure to tell him or her about all the other drugs and supplements you’re taking.
For more information, please click here: http://www.mayoclinic.org/diseases-conditions/hypothyroidism/expert-answers/hypothyroidism/faq-20058536