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 best when your body can absorb it fully—something iron, a mineral commonly taken to treat or prevent anemia can seriously interfere with.

Iron supplements, whether from pills, multivitamins, or even fortified foods, bind to levothyroxine in your gut and stop it from being absorbed. This isn’t a minor issue—it can drop your thyroid hormone levels enough to bring back fatigue, weight gain, cold sensitivity, and brain fog. Studies show that taking iron within four hours of levothyroxine can reduce absorption by up to 55%. That’s not just a small dip—it’s enough to make your treatment ineffective.

It’s not just iron. Calcium, antacids, and even coffee can do the same thing. But iron is one of the most common culprits because so many people, especially women, take it regularly. The fix? Space them out. Take your levothyroxine on an empty stomach, first thing in the morning, with just water. Wait at least four hours before taking iron. Or, if your doctor agrees, take iron at bedtime instead. No guesswork. No hoping it’ll be okay. Just clear timing.

And don’t assume your pharmacist flagged it. Many don’t. You need to ask. Tell your doctor you’re taking iron—whether it’s a standalone supplement, a prenatal vitamin, or even a daily multivitamin. Even if you think it’s harmless, it’s not. Your thyroid dose is fine-tuned based on how well you absorb it. If iron is blocking that, your dose might be too low—and you won’t even know why.

You’re not alone. A lot of people with hypothyroidism also have iron deficiency, especially women with heavy periods or those following plant-based diets. That means you’re stuck between two needs: one to boost your energy with iron, the other to balance your metabolism with levothyroxine. The good news? You don’t have to choose. You just need to space them right.

Below, you’ll find real-life examples, expert tips, and clear comparisons on how to manage this interaction safely. No fluff. No jargon. Just what works.