Levothyroxine Safety: What You Need to Know About Dosage, Side Effects, and Interactions

When you're prescribed levothyroxine, a synthetic form of the thyroid hormone T4 used to treat hypothyroidism. Also known as synthroid, it's one of the most common medications in the U.S.—but it's also one of the most misused. Getting the dose right isn’t just about feeling better. Too little and your energy stays low, your weight won’t budge, and your cholesterol stays high. Too much and you risk heart palpitations, bone loss, or even atrial fibrillation. Levothyroxine safety starts with understanding how your body absorbs it—and what can mess that up.

Thyroid hormone replacement, the medical term for using levothyroxine to restore normal thyroid function isn’t a one-size-fits-all job. Your dose depends on age, weight, other health conditions, and even what time of day you take it. Studies show taking it on an empty stomach, at least 30 to 60 minutes before breakfast, improves absorption. Coffee, calcium supplements, iron pills, and even soy milk can block it. If you take any of these, space them out by at least four hours. That’s not a suggestion—it’s a rule that keeps your treatment working.

Levothyroxine interactions, how other drugs and supplements interfere with thyroid hormone absorption are where most people slip up. Antacids, proton pump inhibitors, and even some cholesterol meds like cholestyramine can reduce how much levothyroxine your body actually uses. Even certain foods like walnuts and high-fiber cereals can do the same. And if you’re on blood thinners like warfarin, levothyroxine can make them stronger—raising your risk of bleeding. That’s why regular blood tests (TSH levels) aren’t optional. You need them every 6 to 8 weeks when starting, then at least once a year after that.

Side effects aren’t always obvious. Many think fatigue or weight gain means their dose is too low—but it could be something else entirely. Anxiety, trouble sleeping, or a racing heart? That’s often a sign you’re on too much. And if you’re over 65 or have heart disease, doctors start with lower doses for a reason. Older adults are more sensitive to the effects, and pushing the dose too high can strain the heart. Levothyroxine isn’t a weight-loss drug, even though some people try to use it that way. That’s dangerous and ineffective.

What you’ll find below isn’t a list of random articles. These are real, practical guides from people who’ve been there—whether it’s managing levothyroxine with other meds, understanding why your dose changed after a lab test, or how to avoid common mistakes that make thyroid treatment fail. No fluff. No hype. Just what works—and what doesn’t—when you’re trying to stay healthy on thyroid medication.