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

When you're taking levothyroxine, a synthetic form of the thyroid hormone T4 used to treat hypothyroidism. Also known as synthroid, it's one of the most commonly prescribed medications in the U.S.—but getting it wrong can cause serious problems. This isn't a drug you can take anytime, anywhere. It needs to be taken on an empty stomach, at least 30 to 60 minutes before breakfast, because food, coffee, and even fiber can block its absorption. If you’re not consistent, your thyroid levels stay unstable, and you’ll keep feeling tired, cold, or depressed—even if you’re taking the right dose.

Levothyroxine interactions, occur when other drugs or supplements interfere with how your body absorbs or processes the hormone. Also known as thyroid medication interactions, they’re more common than most people realize. Calcium supplements, iron pills, antacids, and even soy products can bind to levothyroxine in your gut and stop it from working. Diuretics and certain antidepressants can change how your body clears the hormone, leading to either too little or too much in your bloodstream. Even switching brands—even if they’re both generics—can cause a shift in how your body responds. That’s why your doctor checks your TSH levels every 6 to 8 weeks after any change.

Side effects aren’t always obvious. Too much levothyroxine can mimic hyperthyroidism: rapid heartbeat, weight loss, shaking hands, trouble sleeping. Too little, and you’re back to fatigue, weight gain, and brain fog. Many people assume their symptoms are just aging or stress—but if your dose is off, it’s not in your head. It’s in your thyroid. That’s why timing matters as much as the dose. Taking it at night? Some studies show it works just as well as morning dosing, as long as you’re consistent. But mixing it with food, coffee, or supplements? That’s where things go wrong.

Levothyroxine isn’t a drug you take once and forget. It’s a daily balancing act. You need to track your symptoms, know what to avoid, and communicate clearly with your provider when something feels off. That’s why the posts below cover exactly what you need: how to avoid dangerous interactions, why skipping doses matters, how to talk to your doctor about your meds, and what to do if you think your dose isn’t right. These aren’t theory pages—they’re real-world guides from people who’ve been there.