Short answer: Not at the same time. The Synthroid (levothyroxine) label says calcium carbonate and ferrous sulfate can bind levothyroxine in the gut and lower how much is absorbed, and tells prescribers to keep the doses at least 4 hours apart. NIH fact sheets on calcium and iron repeat that 4-hour gap. Antacids, some cholesterol binders, soy, walnuts and dietary fiber can also lower absorption.
The Synthroid label says to take it once a day on an empty stomach, half an hour to an hour before breakfast, and at least 4 hours before or after drugs known to interfere with its absorption. In its counseling section it says agents such as iron and calcium supplements and antacids can decrease the absorption of levothyroxine, and that patients should not take the tablets within 4 hours of them.
The reason is binding. The label lists calcium carbonate and ferrous sulfate (a common iron supplement) as agents that may bind to levothyroxine, and says this may delay or prevent its absorption and could lead to hypothyroidism, meaning the dose stops doing its job.
| Item | What the label says |
|---|---|
| Calcium carbonate, ferrous sulfate | May bind levothyroxine; take at least 4 hours apart |
| Bile acid sequestrants (colesevelam, cholestyramine, colestipol) and ion exchange resins | Known to lower absorption; take levothyroxine at least 4 hours before them, or monitor TSH |
| Antacids (aluminum and magnesium hydroxides), sucralfate, proton pump inhibitors | May reduce stomach acid needed for absorption; monitor |
| Orlistat | Monitor thyroid function |
| Soybean flour, cottonseed meal, walnuts, dietary fiber | May bind levothyroxine and lower absorption |
| Grapefruit juice | May delay absorption and reduce bioavailability |
The label also says to consider dose changes if levothyroxine is regularly taken within an hour of foods that affect absorption. A fiber supplement such as psyllium counts as dietary fiber. The Xenical (orlistat) label separately says to take levothyroxine and orlistat at least 4 hours apart.
The NIH Office of Dietary Supplements (ODS) calcium fact sheet says calcium carbonate supplements can interfere with levothyroxine absorption and points to the label's instruction not to take levothyroxine within 4 hours of the supplement.
The ODS iron fact sheet says taking iron and levothyroxine at the same time can cause clinically significant drops in how well levothyroxine works in some people, and that the labels for some levothyroxine products advise against taking it within 4 hours of iron. Multivitamins and prenatal vitamins often contain both calcium and iron, so the same timing question applies to them.
The label says the dose is adjusted using TSH (and free T4 in some cases), and that the full effect of a dose change may take 4 to 6 weeks. If the gap between levothyroxine and a calcium or iron supplement changes, absorption can change too, which may show up at the next blood test rather than right away.
Your prescriber or pharmacist can say how to fit levothyroxine and supplements into your day. Things worth noting before a thyroid test: when you take levothyroxine, what else you take in the 4 hours around it, any new supplement, and any biotin, which the label says to stop at least 2 days before TSH or T4 testing. Dose Track can remind you of each pill at its own time and keep your TSH results next to your log.
Read more on Dose Track: Synthroid side effects · Levothyroxine guide · Calcium supplements · Iron supplements · TSH (glossary) · Do GLP-1 medicines affect other pills?