Levothyroxine and Iron Interaction: What You Need to Know

When you take levothyroxine, a synthetic thyroid hormone used to treat hypothyroidism. Also known as Synthroid, it works by replacing the hormone your thyroid doesn’t make enough of. But if you’re also taking iron supplements, commonly used to treat anemia or low iron levels, you might be quietly sabotaging your treatment. Iron doesn’t just sit there—it binds to levothyroxine in your gut, stopping your body from absorbing it. That means your thyroid levels stay low, even if you’re taking your pill every day.

This isn’t a rare issue. It’s one of the most common reasons thyroid patients don’t feel better. The same problem happens with calcium, antacids, and even some fiber supplements. But iron is the biggest offender. Studies show taking iron just four hours before or after levothyroxine can cut absorption by up to 50%. That’s like taking half a pill and wondering why your energy is still gone. You might think you’re doing everything right—taking your pill on an empty stomach, waiting an hour before breakfast—but if you’re also swallowing an iron tablet at breakfast, you’re undoing all that effort.

So what’s the fix? Simple: space them out. Take your levothyroxine first thing in the morning, at least 60 minutes before eating or drinking anything but water. Then wait at least four hours before taking iron. If you take iron at night, that’s often the easiest window—take levothyroxine in the morning, iron before bed. No guesswork. No confusing charts. Just timing that works. And if you’re on a multivitamin with iron? Check the label. Many do. Switch to one without iron, or take it at a different time.

This interaction isn’t just about numbers on a lab report. It’s about how you feel. Fatigue. Brain fog. Weight gain. Cold hands. These aren’t just "bad days"—they’re signs your thyroid meds aren’t working because iron is blocking them. And if you’re a woman with heavy periods, postpartum, or managing menopause, you’re more likely to need iron. That makes this interaction even more critical to get right.

You’ll find real-life stories and clear guides below on how to manage this interaction without sacrificing your health. Some posts dive into how other supplements like calcium or magnesium affect levothyroxine. Others show you how to track your thyroid levels over time so you know your meds are actually working. You’ll also see how people balance iron needs with thyroid treatment—without constant guesswork or side effects. This isn’t theory. It’s what works for real people who’ve been where you are.