You can take antacids if you’re on levothyroxine, but you need to separate them by at least four hours. Taking them together, or even close together, allows the antacid to bind to the thyroid hormone in your stomach and prevent your body from absorbing it. This isn’t a minor interaction. In clinical studies, patients who took antacids alongside levothyroxine saw their TSH levels nearly triple, meaning their thyroid medication was failing to do its job.
Why Antacids Block Levothyroxine
Levothyroxine is absorbed in your gut, and it needs an acidic stomach environment to dissolve properly. Antacids interfere with this process in two ways. First, they raise the pH in your stomach, making the environment less acidic. Second, and more importantly, the active ingredients in antacids physically bind to levothyroxine molecules, forming clumps that your intestines can’t absorb. These bound complexes simply pass through your digestive system and get eliminated, taking your thyroid dose with them.
This binding happens with all the common antacid types. Aluminum hydroxide (found in Maalox and Mylanta) has a strong nonspecific adsorptive capacity for levothyroxine. Calcium carbonate (Tums) forms insoluble chelates with the hormone at the acidic pH found in a normal stomach. Magnesium-based antacids also bind to levothyroxine, with magnesium carbonate appearing to be a stronger binder than magnesium oxide. No matter which brand or type of antacid you reach for, the interaction risk is real.
How Much It Actually Affects Your Thyroid Levels
The impact is large enough to push a well-managed patient back into hypothyroidism. In one study, five patients whose thyroid levels were stable on levothyroxine were given aluminum hydroxide antacid tablets four times daily for two to four weeks. Their average TSH rose from 2.62 to 7.19 mU/L. To put that in perspective, the standard treatment target for most people on levothyroxine is a TSH between 0.5 and 2.0 mU/L. A jump to 7.19 means the medication is being significantly undermined.
What makes this interaction tricky is that the symptoms creep in gradually. You wouldn’t feel a sudden change the way you might with a missed dose. Instead, over days or weeks of regular antacid use, you might notice increasing fatigue, feeling colder than usual, sluggish thinking, or unexplained weight gain. These are the hallmark signs of underactive thyroid function returning, and many people attribute them to stress or aging rather than connecting them to that bottle of Tums on the nightstand.
The Four-Hour Rule
The Mayo Clinic recommends taking antacids (including aluminum hydroxide, magnesium hydroxide, simethicone, and calcium carbonate products like Tums) at least four hours before or four hours after your levothyroxine dose. This applies to brand-name products like Maalox, Mylanta, and any generic equivalents.
The simplest way to manage this is to stick with the standard advice of taking levothyroxine first thing in the morning on an empty stomach, then waiting at least four hours before using any antacid. If you need an antacid at night for heartburn, that’s usually fine since most people take their levothyroxine in the morning and the gap naturally exceeds four hours. The issue arises when people pop antacids throughout the day, especially around breakfast time.
Other Acid-Related Medications Matter Too
Antacids aren’t the only acid-related drugs that interact with levothyroxine. Proton pump inhibitors (like omeprazole or lansoprazole) and H2 blockers work differently from traditional antacids. They don’t physically bind to levothyroxine the way calcium or aluminum does. Instead, they suppress acid production, which can reduce how well levothyroxine dissolves and gets absorbed. The same four-hour spacing rule applies to these medications as well.
Iron supplements, cholesterol-lowering resins, and calcium supplements also bind to levothyroxine through similar mechanisms. If you’re juggling multiple medications and supplements alongside your thyroid hormone, it’s worth mapping out a schedule that keeps levothyroxine isolated in its own window, ideally first thing in the morning with nothing but water.
Signs the Interaction Is Affecting You
If you’ve been taking antacids regularly and you’re on levothyroxine, watch for the return of hypothyroid symptoms: persistent tiredness despite adequate sleep, feeling unusually cold, constipation, dry skin, brain fog, or unexplained weight gain. A simple TSH blood test will confirm whether your thyroid levels have drifted. If your TSH has climbed above your usual range and you’ve recently started using antacids more frequently, the interaction is the likely culprit.
Fixing the problem is usually straightforward. Separating the two medications by four hours typically restores normal absorption without needing a dose adjustment. In some cases where spacing is impractical, a healthcare provider may increase the levothyroxine dose to compensate, though proper timing is the preferred solution. Liquid levothyroxine formulations may also be less susceptible to binding interactions, though they aren’t necessary for most people who can simply adjust their schedule.