Can iron or calcium interfere with levothyroxine?

What happens when minerals and levothyroxine are taken together?

Some iron and calcium products can bind to levothyroxine or otherwise reduce its absorption in the gut, so less medicine reaches the body.23

In a 1992 study without a control group, 14 people with an underactive thyroid took ferrous sulfate and their thyroid medicine together for 12 weeks. Their average thyroid-stimulating hormone (TSH), a blood marker used to assess thyroid treatment, rose from 1.6 to 5.4 mU/L. The free thyroxine index, an estimate of available thyroid hormone, did not change significantly. A higher TSH can prompt a treatment review, but this small study cannot predict an individual result. A separate small study examined calcium carbonate.23

How should someone manage the timing?

The right plan depends on your prescription, supplements, other medicines and routine. The US Synthroid tablet label revised in 2024 says to take that product at least four hours apart from iron or calcium supplements. It also instructs people to take those tablets on an empty stomach before breakfast. This is one named US tablet label, not a universal schedule for every formulation or country.5

The UK NHS also advises checking the package and asking a pharmacist or doctor about interacting products. Do not change prescribed treatment based on an example.1

What about breakfast, coffee or a multivitamin?

The UK NHS advises taking levothyroxine on an empty stomach 30–60 minutes before the first meal or a caffeinated drink such as tea or coffee. The US Synthroid tablet label gives a before-breakfast instruction for that product. Other directions can differ. Check your leaflet and discuss a workable routine with a pharmacist or prescriber.15

Iron and calcium may be in a multivitamin or prenatal supplement; some antacids also contain calcium. Check the ingredients, not just the front of the pack. Bring the names and labels of everything you use. See how to read a supplement label and why food instructions differ.7

Is biotin the same kind of interaction?

No. Biotin, found in some hair and nail products, can affect certain thyroid blood-test results. This is a measurement problem, different from a mineral reducing medicine absorption. Depending on the laboratory method, a result can appear higher or lower than it really is; not every method is affected.6

A 2021 study compared laboratory methods after 13 adults took a specified high amount of biotin for eight days. Some thyroid-related results changed on particular platforms and at particular sampling times. This small study does not show that every supplement changes every test or establish a personal waiting period.6

Tell the clinician and laboratory about all supplements before tests. Ask for instructions for your test and product. See biotin and blood tests.

What if I am pregnant or planning pregnancy?

The UK NHS says levothyroxine can be taken during pregnancy when needed and additional thyroid blood tests may be required. Contact your prescribing clinician promptly if you are planning pregnancy, pregnant or become pregnant. Follow any individual plan already provided, and discuss your prenatal supplement and its ingredients. This article cannot decide your plan; do not stop prescribed thyroid medicine on your own.189

What else should a reader consider?

Include separate tablets, multivitamins and fortified products when making a list of what is used. A healthcare professional may interpret thyroid tests alongside the medicine routine, symptoms and other causes; do not adjust a dose or stop needed treatment based only on this article. For related questions see iron, absorption and deficiency and other supplement–medicine interactions.1

After starting levothyroxine or changing its dose, follow-up blood tests help the prescriber assess the response. UK Specialist Pharmacy Service guidance describes checking TSH from six weeks after a start or dose change; the timing and further tests depend on the person and their care plan. Tell the prescriber if you start, stop or change the timing of an iron or calcium product, so the routine and need for follow-up can be reviewed.10

Orlistat, a weight-management medicine, raises another question: the US Synthroid label advises monitoring thyroid function when the two are used together. The September 2026 US Xenical label also says not to take Xenical and levothyroxine at the same time and to separate them by at least four hours. Its effect on vitamin A absorption is a separate issue; ask the prescribing clinician about the exact products.512

If your levothyroxine product changes and you notice symptoms, tell your prescriber or pharmacist. The UK medicines regulator says generic prescribing remains appropriate for most people, but a small proportion report symptoms after a tablet change; a clinician may consider thyroid tests and, if symptoms persist, a consistent product. Symptoms can have other causes. Do not stop treatment on your own.11