Levothyroxine
This is an educational list of interactions tied to a public source. It is not a complete medication review and it is not medical advice. Ask a pharmacist, or the clinician who prescribes the medicine, before you start, stop, or change a supplement. A pair that is missing here is not a safety clearance. SupplementScore takes no ads, sponsorships, or affiliate income. How this site is funded.
Levothyroxine (Synthroid, Levoxyl, and similar tablets) is easy to block if a mineral is in the stomach at the same time. Biotin does not block the pill. It can make a thyroid blood test read wrong.
The iron and calcium advice is about separating doses, not about never using those nutrients. Someone who is iron deficient still needs a plan. The plan has to come from the prescriber or a pharmacist.
Names people search: Levothyroxine, Synthroid, Levoxyl, Tirosint, Unithroid
Separate the doses
NIH ODS says taking iron and levothyroxine at the same time can meaningfully reduce how well levothyroxine works in some people. Labels for these thyroid tablets warn that iron supplements can reduce absorption and say not to take levothyroxine within 4 hours of an iron supplement. You can still treat iron deficiency. The doses have to be separated, and the thyroid dose may need a recheck.
Separate the doses
NIH ODS says calcium carbonate supplements can interfere with absorption of levothyroxine. The FDA-approved label tells patients not to take levothyroxine within 4 hours of a calcium carbonate supplement. The cited instruction names calcium carbonate specifically. If your bottle is a different calcium salt, ask a pharmacist rather than assuming it is free of this problem.
Can skew a lab test
NIH ODS says biotin supplements above the recommended amount may cause false results on some lab tests, including tests that measure hormones such as thyroid hormone. Biotin does not stop levothyroxine from being absorbed. It can make TSH or other hormone results look wrong, which can lead to a wrong dose change. Tell the lab and the prescriber that you take biotin before a thyroid test. The FDA has also warned that biotin can interfere with some lab tests more broadly, including some troponin tests used to detect a heart attack.