Levothyroxine Side Effects: What You Need to Know Before Taking It

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 pills in the U.S. for people with underactive thyroids. But taking it doesn’t mean everything just clicks into place. Many people feel better right away, but others notice strange symptoms—racing heart, shaky hands, trouble sleeping—and wonder if it’s the medicine or something else.

Levothyroxine side effects often come down to dosage. Too little, and you stay tired, cold, and sluggish. Too much, and your body starts acting like it’s overworked—heart palpitations, weight loss without trying, anxiety, even bone thinning over time. The thyroid hormone replacement, the medical approach to restoring normal thyroid function using synthetic hormones isn’t one-size-fits-all. Your doctor doesn’t just pick a number; they adjust based on your age, weight, other meds, and how your blood tests respond. That’s why skipping blood work or changing your dose on your own can backfire.

Some side effects aren’t about the dose at all. If you take levothyroxine with coffee, calcium supplements, or iron pills, your body might not absorb it properly. That’s not a side effect—it’s a mistake in timing. And if you’re on other drugs like blood thinners or antidepressants, levothyroxine can interact in ways that aren’t obvious. Even something as simple as switching brands can throw your levels off. Generic versions are fine, but they’re not always identical in how your body handles them.

Most people take levothyroxine for life. That means learning how to spot the early warning signs: Is your heart skipping beats more than usual? Are you losing weight even though you’re eating the same? Do you feel jittery when you used to feel calm? These aren’t normal. They’re your body saying, "Something’s off." And the fix isn’t always more medicine—it might be less, or a change in when you take it.

What you’ll find below are real stories and facts from people who’ve been there. We’ve pulled together posts that break down what levothyroxine side effects actually look like in practice, how they connect to other conditions like heart issues or bone loss, and what steps you can take to stay in control. No fluff. Just what matters when you’re trying to feel like yourself again.