Can calcium or iron affect levothyroxine?

Absorption and test interference are different problems

Products such as calcium carbonate and ferrous sulfate may reduce how much levothyroxine is absorbed in the digestive tract. Even when the dose on the box stays the same, less of the medicine may enter your body. Biotin, also called vitamin B7, can interfere with certain thyroid function tests, so the reported values may differ from your actual levels. Your doctor needs to investigate these two possibilities separately. [1–3]

Check the ingredients on the back of multivitamin packages, too. A front label that says “hair and nail support” or “prenatal nutrition” will not tell you whether the product contains calcium, iron, or biotin. One bottle can contain several ingredients. A photo of the full label makes it easier for your doctor to check than the brand name alone. [2,3]

How doctors read the two values on your report

Thyroid-stimulating hormone (TSH) is an important measure when monitoring thyroid hormone replacement for primary hypothyroidism. Reports often include free thyroxine (FT4) as well. Your doctor reads these values alongside previous reports and how you actually take your medicine. Central hypothyroidism is monitored differently and calls for a different interpretation. [1,2]

If you had blood drawn at different hospitals, bring both original reports with the dates, units, and reference ranges. If you copy only two numbers into your phone, the doctor still needs to ask whether they refer to the same type of test and which laboratory produced them. If a result differs from what the doctor expected, they will also check whether you recently started a different box of medication or added a supplement, or whether something may have interfered with the test. [1–3]

How to check medicine you bought outside China

For a follow-up visit in China, bring your original prescription and photos of the front and back of the medicine box. Ask your doctor or pharmacist to check the generic name, dosage form, strength per tablet, and manufacturer. Note the date you actually started taking medicine from that box. Even if both boxes say levothyroxine sodium, tell your doctor if the brand or manufacturer changed. The American Thyroid Association's patient guide recommends reporting such changes so your doctor can decide whether you need another test. [3]

Bring the packaging and a record of how you take each product

• Boxes and ingredient lists. List levothyroxine, calcium, iron, multivitamins, and other medicines you take regularly as separate items. Keep the strength per tablet or the ingredient list.
• A record of what you actually take. Note when you have breakfast and when you take each product. Include missed doses, temporary changes in timing, and products you only recently started.
• Recent and earlier thyroid function reports. Also note roughly when you started a different box of medicine or added a supplement so your doctor can compare those dates with your results.

You can ask your doctor, “Should we first look at how I take the medicine and whether absorption is an issue, or first check whether biotin affected the test?” Your treating doctor uses your medicines and test results to decide how you should take your medication, whether to change anything, and when to repeat testing. [1–3]