Lamotrigine and Thyroid Function: What You Need to Know

Many people taking lamotrigine for epilepsy or bipolar disorder never think about their thyroid. But if your mood swings aren’t improving, or you’re feeling unusually tired, cold, or gain weight without reason, your thyroid might be the hidden culprit. Lamotrigine doesn’t directly damage the thyroid, but it can interfere with how your body uses thyroid hormones-and that’s something your doctor needs to watch.

How Lamotrigine Affects Thyroid Hormones

Lamotrigine is an antiepileptic drug, also used as a mood stabilizer. It works by calming overactive nerve signals in the brain. But the liver enzymes it affects-specifically CYP3A4 and UGT1A4-are the same ones that help break down thyroid hormones like T3 and T4. When lamotrigine speeds up this process, your body clears thyroid hormones faster than normal. That can lead to lower levels in your blood, even if your thyroid gland is working fine.

This isn’t always obvious. Many people feel fine at first. But over months, subtle changes creep in: fatigue that won’t go away, dry skin, constipation, or a drop in energy that feels like depression returning. These aren’t signs that your bipolar disorder is worsening-they could be signs your thyroid is underactive.

A 2023 study in the Journal of Clinical Psychopharmacology tracked 187 people on long-term lamotrigine. Nearly 30% showed a drop in free T4 levels within six months, even though their TSH (the thyroid-stimulating hormone) stayed normal. That’s important because most doctors only check TSH. If TSH looks okay, they assume the thyroid is fine. But lamotrigine can cause what’s called “euthyroid sick syndrome”-normal TSH, low T4. That’s why just checking TSH isn’t enough.

What Symptoms to Watch For

If you’re on lamotrigine and notice any of these, don’t ignore them:

• Constant tiredness, even after a full night’s sleep
• Feeling cold when others are comfortable
• Unexplained weight gain (5+ pounds in a few months)
• Dry skin or hair loss
• Memory fog or trouble concentrating
• Depression symptoms returning despite stable medication

These aren’t normal side effects of lamotrigine like dizziness or headaches. They’re red flags for thyroid disruption. And they’re often mistaken for the original condition you’re treating-like bipolar depression. That’s why so many people stay on higher doses of mood stabilizers when what they really need is thyroid support.

Testing Thyroid Levels While on Lamotrigine

If you’re on lamotrigine, ask for a full thyroid panel-not just TSH. Here’s what you need:

• Free T4-measures active thyroid hormone in your blood
• Free T3-the most active form your cells use
• TSH-still important, but not enough on its own
• Thyroid Peroxidase Antibodies (TPOAb)-rules out autoimmune thyroid disease like Hashimoto’s

Don’t wait for symptoms to get worse. Get tested before you start lamotrigine, then again at 3 months, 6 months, and annually after that. Some doctors skip this unless you’re clearly symptomatic. But if you’re feeling off, push for it. A simple blood test can save you months of unnecessary medication changes.

What Happens If Thyroid Levels Drop?

If your free T4 drops below the normal range, your doctor might suggest a low dose of levothyroxine (Synthroid). This isn’t about adding another drug-it’s about fixing an imbalance caused by lamotrigine. Many patients report feeling like themselves again within weeks: energy returns, brain fog lifts, mood stabilizes.

One patient in a 2024 case series from the University of Auckland started on lamotrigine for bipolar II. After four months, her mood worsened. Her doctor assumed it was treatment resistance. She added a higher dose of lamotrigine-and her fatigue got worse. Only after she requested a full thyroid panel did they find her free T4 was 0.7 ng/dL (normal range: 0.8-1.8). She started on 25 mcg of levothyroxine. Within six weeks, her mood improved, her energy came back, and her lamotrigine dose was safely lowered.

That’s not rare. It’s underdiagnosed.

Can You Prevent This?

Yes, but it takes action. Here’s what works:

1. Get baseline thyroid tests before starting lamotrigine
2. Ask for free T4 and free T3-not just TSH
3. Test again at 3 and 6 months, then yearly
4. Track your symptoms in a journal: energy, weight, mood, sleep
5. If you feel off, don’t assume it’s your mental health-it might be your thyroid

Some people never have issues. Others need a tiny thyroid boost. There’s no way to predict who will be affected. That’s why testing is the only reliable method.

What About Other Medications?

Lamotrigine isn’t the only drug that affects thyroid function. Lithium, valproate, and even some antidepressants like SSRIs can interfere too. If you’re on more than one medication, the risk multiplies. Lithium, for example, can block thyroid hormone release. Valproate can lower T4 levels. Combine those with lamotrigine’s enzyme effects, and your thyroid gets squeezed from multiple sides.

If you’re on multiple mood stabilizers, your thyroid testing schedule should be even more aggressive. Every 3 months for the first year, then every 6 months. Don’t assume your doctor will bring it up. Bring it up yourself.

What If Your Doctor Says It’s Not a Problem?

If your doctor dismisses your concerns because your TSH is normal, ask them this: “If my free T4 is low, does that mean I need treatment-even if TSH is normal?”

Some doctors still believe TSH is the only test that matters. But endocrinologists and psychiatric pharmacologists know better. The American Thyroid Association and the International Society for Bipolar Disorders both recommend checking free T4 in patients on lamotrigine, especially if they have mood symptoms.

If your doctor won’t order the full panel, ask for a referral to an endocrinologist. Or get a private test-many labs offer direct-to-consumer thyroid panels for under $100. You deserve to feel well. Don’t let outdated guidelines stand in your way.

Final Thoughts

Lamotrigine is a powerful tool for managing epilepsy and bipolar disorder. But like all medications, it has hidden effects. Thyroid disruption is one of the most common-and easiest to fix. You don’t need to suffer in silence. If you’ve been feeling off for months and no one can explain why, thyroid function is a place to look. A simple blood test can change everything.

Don’t wait for a crisis. Get tested. Track your symptoms. Speak up. Your brain and your thyroid are connected-and both deserve attention.