Levothyroxine Dose: What You Need to Know About Dosage, Adjustments, and Safety

When your thyroid doesn’t make enough hormone, levothyroxine, a synthetic version of the thyroid hormone T4 used to treat hypothyroidism. Also known as synthroid, it’s one of the most prescribed drugs in the U.S.—but getting the dose right isn’t one-size-fits-all. Too little and you’ll stay tired, gain weight, and feel cold. Too much and you might get heart palpitations, trouble sleeping, or even bone loss over time.

The starting dose usually depends on your age, weight, and how low your thyroid levels are. For a healthy adult under 65, doctors often begin with 50 to 75 micrograms per day. But if you’re older, have heart disease, or have severe hypothyroidism, they might start lower—like 25 mcg—to avoid stressing your heart. Your TSH levels, a blood test that measures thyroid-stimulating hormone from the pituitary gland are the main guide. Most people need a TSH between 0.5 and 3.0 mIU/L, but some feel best at the lower end of that range. It can take 4 to 6 weeks after a dose change for your TSH to stabilize, so don’t rush adjustments.

Other drugs can mess with how levothyroxine works. calcium supplements, including antacids and iron pills can block absorption if taken at the same time. That’s why you’re told to take levothyroxine on an empty stomach, at least 30 to 60 minutes before breakfast. Even coffee, soy milk, and high-fiber meals can interfere. And if you’re on blood thinners like warfarin, levothyroxine can make them stronger—raising your risk of bleeding. That’s why your doctor checks your INR if you’re on both.

Many people stay on levothyroxine for life, but doses can shift with weight changes, pregnancy, or new medications. Pregnant women often need more—sometimes up to 30% higher—because their bodies process the hormone faster. And if you’re switching brands, even if they’re both generic, your TSH might dip or rise. That’s why consistency matters: stick with the same manufacturer unless your doctor says otherwise.

There’s no magic number for levothyroxine dose. It’s not just about your lab results—it’s about how you feel. If you’re still exhausted, gaining weight, or depressed despite a "normal" TSH, your doctor might look at free T4 levels or consider a different approach. And if you’re taking it with other thyroid meds like liothyronine (T3), that’s a whole different conversation.

Below, you’ll find real-world advice from posts that cover everything from how to avoid accidental double-dosing to how kidney or liver issues affect thyroid meds. You’ll see what doctors actually do when doses go wrong, how other drugs interact, and why monitoring isn’t just a formality—it’s your safety net.