Iron and Levothyroxine Timing: What You Need to Know

When you take levothyroxine, a synthetic thyroid hormone used to treat hypothyroidism. Also known as Synthroid, it needs to be absorbed properly to work at all. Now add iron supplements, a common treatment for low iron or anemia. Also known as ferrous sulfate, it binds to levothyroxine in your gut and stops your body from absorbing the thyroid medicine. That’s not a minor issue—it can mean your thyroid levels stay low even if you’re taking your pill every day.

This isn’t theory. People on both meds often feel tired, gain weight, or get cold easily—not because their dose is wrong, but because iron is blocking absorption. Studies show taking iron within four hours of levothyroxine can drop thyroid hormone levels by up to 55%. That’s why doctors tell you to space them out. The rule? Take levothyroxine on an empty stomach, at least 30 to 60 minutes before breakfast. Wait at least four hours before taking iron. If you take iron at night, do it right before bed, and make sure your last meal was at least two hours earlier. No coffee, no calcium, no antacids—those mess with absorption too.

You might be wondering if chewable iron or liquid forms are safer. They’re not. All iron supplements—whether pills, drops, or gummies—contain the same active ingredient that binds to levothyroxine. Even if your iron is part of a multivitamin, skip it on days you take your thyroid pill. Same goes for calcium, magnesium, or zinc supplements. They all interfere. If you’re taking multiple supplements, write them down. Track when you take each one. Use a phone alarm if you have to. This isn’t about being perfect—it’s about being consistent. One day you take iron with breakfast, the next you forget, and your thyroid levels swing. That’s how you end up back in the doctor’s office for a blood test.

Some people try to fix this by switching to a different thyroid med. But even natural desiccated thyroid or brand-name versions like Tirosint still get blocked by iron. The problem isn’t the pill—it’s the iron. And if you’re a woman over 50, postpartum, or have heavy periods, you’re more likely to need both. That’s why this timing issue comes up so often. It’s not rare. It’s routine.

There’s one exception: if your doctor prescribes an IV iron infusion, that’s different. It goes straight into your bloodstream, so it doesn’t touch your gut. No interference. But if you’re swallowing pills, spacing is non-negotiable. Don’t guess. Don’t hope. Test your levels after you’ve been consistent for six weeks. If your TSH is still high, check your supplement schedule before asking for a higher dose.

Below, you’ll find real posts from people who’ve been there—how they figured out the right timing, what worked, what didn’t, and how they stopped feeling exhausted all the time. No fluff. Just what you need to make this work.