Lithium and Thyroid Dysfunction: What You Need to Know

When you take lithium, a mood-stabilizing drug commonly used for bipolar disorder. Also known as lithium carbonate, it helps control extreme mood swings—but it doesn’t come without trade-offs. One of the most common and often overlooked side effects is its impact on the thyroid, a small gland in your neck that controls metabolism, energy, and body temperature. About 20% to 50% of people on long-term lithium develop some form of thyroid dysfunction, mostly hypothyroidism.

Here’s how it works: lithium interferes with the thyroid’s ability to release hormones like T3 and T4. It doesn’t destroy the gland, but it blocks the release of stored hormones and can reduce the gland’s sensitivity to TSH, the signal from your brain that tells the thyroid to produce more. Over time, this leads to low hormone levels, which means your body slows down. You might feel tired, gain weight, get cold easily, or notice your memory getting foggy. These symptoms are easy to blame on stress, aging, or even the bipolar disorder itself—until your doctor runs a simple blood test.

That’s why regular thyroid function tests, including TSH, free T4, and sometimes free T3 levels are non-negotiable if you’re on lithium. Doctors usually check these before you start, then every 6 to 12 months after that. If your TSH is high and T4 is low, you’ve got hypothyroidism. The good news? It’s usually easy to fix with a daily thyroid hormone pill like levothyroxine. You don’t have to stop lithium. Most people keep taking it and just add a tiny pill to balance things out.

Some people worry that thyroid problems mean they can’t stay on lithium. That’s not true. With proper monitoring, lithium remains one of the most effective tools for managing bipolar disorder. The key is catching changes early. If you’ve been on lithium for more than a year and suddenly feel off—like you’re dragging through the day or your mood is harder to control—ask for a thyroid check. It’s quick, cheap, and could explain everything.

You’re not alone in this. Many people on lithium deal with thyroid issues and still live full, stable lives. What matters isn’t avoiding the side effect—it’s managing it smartly. Below, you’ll find real-world insights from people who’ve been there: how symptoms showed up, what tests actually revealed, and how treatment changed their day-to-day. These aren’t theory pages. They’re practical stories from the front lines of long-term medication use.