Thyroid medicines can interact with other drugs, supplements and foods. Most interactions are manageable with timing or dose adjustment, but they need to be anticipated.
Levothyroxine absorption
Levothyroxine is absorbed in the small intestine, and several substances reduce absorption:
- Calcium and iron supplements — separate by at least four hours
- Antacids and proton pump inhibitors — can reduce absorption
- Multivitamins containing minerals
- Soy products, coffee and high-fibre foods — can reduce absorption if taken at the same time
- Bile acid sequestrants — separate by several hours
Taking levothyroxine on an empty stomach, 30–60 minutes before food, minimises these effects.
Medicines that affect thyroid function
Some medicines can alter thyroid tests or thyroid function:
- Amiodarone — rich in iodine; can cause over- or underactivity
- Lithium — can cause goitre and hypothyroidism
- Interferon — can trigger thyroiditis
- Immune checkpoint inhibitors — commonly cause thyroid dysfunction
- Some contrast dyes — high iodine load
If you start one of these, your clinician may monitor thyroid function more closely.
Antithyroid drugs
Carbimazole, methimazole and propylthiouracil can rarely cause side effects, including liver problems and a low white cell count. Seek urgent advice for fever, sore throat, mouth ulcers or jaundice while taking them.
They can also interact with warfarin and other medicines, so tell your prescriber about all drugs you take.
Supplements and thyroid tests
Biotin (vitamin B7) can interfere with many thyroid immunoassays, producing falsely abnormal results. Stop biotin a few days before thyroid blood tests unless advised otherwise.
Iodine and kelp supplements can worsen thyroid disease and should not be taken without medical advice.
What to do
- Keep an up-to-date list of all medicines and supplements.
- Tell every prescriber that you take thyroid medication.
- Ask about timing when starting a new medicine.
- Do not stop or change thyroid medication without advice.
The bottom line
Most thyroid medication interactions can be managed with correct timing and monitoring. The key is to be transparent about everything you take and to ask before making changes.