When you're taking levothyroxine, a synthetic thyroid hormone used to treat hypothyroidism. Also known as Synthroid, it's one of the most commonly prescribed medications in the world. and also need iron, a mineral essential for making red blood cells and carrying oxygen. Often taken as ferrous sulfate or other iron supplements to treat anemia., timing matters more than you might think. These two don’t just sit quietly side by side in your body—they actively interfere with each other. If you take them together, your body might absorb less of the thyroid hormone, making your treatment less effective. This isn’t a guess or a rumor. It’s backed by clinical studies and confirmed by pharmacists and endocrinologists.
The problem isn’t that iron is bad or levothyroxine is fragile. It’s about chemistry. Iron binds to levothyroxine in your gut, forming a complex your body can’t break down. That means the thyroid hormone passes through without being absorbed. The same thing happens with calcium, antacids, and even some fiber supplements. You don’t have to stop taking iron. You just need to space it out. The standard advice? Take levothyroxine on an empty stomach, first thing in the morning, with water. Wait at least four hours before taking iron. Or flip it: take iron at bedtime, at least four hours after your last dose of thyroid meds. Many people find the nighttime option easier to stick with. If you’re on multiple supplements, write down your schedule. Use a pill organizer with labeled times. Set phone alarms. Missing one dose won’t break your treatment, but doing it wrong every day slowly lowers your thyroid levels—without you even noticing.
And it’s not just about timing. Some iron supplements are worse than others. Ferrous sulfate is the cheapest and most common, but it’s also the most likely to cause stomach upset and interfere with absorption. Ferrous fumarate or ferrous gluconate might be gentler and just as effective. Talk to your doctor or pharmacist about which form you’re taking. If you’re on a multivitamin with iron, skip it on days you take levothyroxine. Same goes for antacids, proton pump inhibitors, and even coffee or soy milk—they all mess with absorption. Your thyroid medication needs a clean path. Keep it simple: water, empty stomach, wait four hours. That’s it.
People often don’t realize their fatigue, brain fog, or weight gain isn’t from their thyroid condition returning—it’s from how they’re taking their meds. A 2021 study in the Journal of Clinical Endocrinology & Metabolism found that nearly 30% of patients with poorly controlled hypothyroidism were taking iron or calcium within two hours of their levothyroxine. Fix the timing, and their TSH levels dropped back into range—no dose change needed. That’s the power of small, smart habits.
Below, you’ll find real-world advice from people who’ve been there, guides on how to read your prescription labels, and clear comparisons of what works and what doesn’t when managing multiple medications. No fluff. Just what you need to take control.
Calcium and iron supplements can reduce levothyroxine absorption by up to 36%, leading to poor thyroid control. Learn the right timing to avoid interactions, which foods and drinks interfere, and how to fix your routine for better results.