Thyroid Medications in Pregnancy: Dose Adjustments and Monitoring

When you find out you're pregnant, your body changes in ways you never expected. One of the most important, yet often overlooked, shifts happens in your thyroid. If you're already taking thyroid medication-most likely levothyroxine-your dose isn't going to stay the same. In fact, it probably needs to go up. And not just a little. Many women need a 30% increase in their dose within weeks of conception. This isn't optional. It's critical for your baby’s brain development.

Why Your Thyroid Needs More Support During Pregnancy

Your thyroid makes hormones that control metabolism, energy, and, most importantly for pregnancy, fetal brain growth. Before 12 weeks, your baby can't make its own thyroid hormone. It relies entirely on yours. If your levels drop-even slightly-your baby’s neurodevelopment could be affected. Studies show that untreated or poorly managed hypothyroidism in pregnancy increases the risk of miscarriage by up to 60%, preterm birth, and lower IQ scores in children by 7 to 10 points.

The good news? Levothyroxine is safe. It’s classified as FDA Pregnancy Category A, meaning no evidence of harm in human studies. The real issue isn’t the medication-it’s the timing and dose. Most women with pre-existing hypothyroidism need a higher dose almost immediately after conception. Yet, many don’t get adjusted until their first prenatal visit at 8 to 10 weeks. That’s too late.

How Much More Do You Need?

There’s no one-size-fits-all answer, but here’s what the data says:

• If you have pre-existing hypothyroidism, increase your dose by 20-30% as soon as you confirm pregnancy. That’s about 12.5 to 25 mcg extra per day for most people.
• For women with TSH over 10 mIU/mL at diagnosis, start at 1.6 mcg/kg/day. For TSH under 10, start at 1.0 mcg/kg/day.
• A 2021 NIH study of 280 pregnant women found their average dose jumped from 85.7 mcg pre-pregnancy to 100.0 mcg in the first trimester-a 16.7% increase.
• Some guidelines, like ACOG’s, suggest a 50 mcg jump right away for women already on medication. Others recommend smaller, stepwise increases.

Why the variation? Because every body is different. Your weight, how long you’ve had hypothyroidism, and whether you have thyroid antibodies all matter. But the bottom line: don’t wait. If you’re planning pregnancy or just found out you’re pregnant, talk to your doctor about adjusting your dose now-not next month.

When to Check Your TSH

Checking your TSH (thyroid-stimulating hormone) is the only reliable way to know if your dose is right. TSH levels change fast in early pregnancy, so timing matters.

• Test within 4 weeks of confirming pregnancy.
• Test every 4 weeks until your TSH stabilizes.
• Then check again at 24-28 weeks and 32-34 weeks.

Some OB/GYNs don’t test at all until the second trimester. That’s risky. Research shows 75% of women need dose adjustments in the first trimester. Waiting means your baby might be missing out on vital thyroid hormone during the most critical window.

Target TSH levels also vary by guideline:

• American Thyroid Association (ATA): ≤2.5 mIU/mL throughout pregnancy
• Endocrine Society: ≤2.5 mIU/mL in first trimester, up to 3.0 mIU/mL in second and third

Here’s what happens if you go over: women with TSH above 2.5 in the first trimester have a 69% higher risk of miscarriage. That’s not a small number. It’s a signal that your dose needs attention.

How to Take Your Medication Right

Levothyroxine works best when taken on an empty stomach. Take it first thing in the morning, wait 30 to 60 minutes before eating or drinking anything besides water. Avoid calcium, iron, and prenatal vitamins within 4 hours of your dose. These can block absorption by 35-50%.

Some patients try to manage the increased dose by taking extra pills on weekends-say, two doses on Saturday and Sunday. But that can cause spikes and dips in hormone levels. A better approach? Spread the extra dose across the week. For example, if you need 25 mcg more per day, take your normal dose Monday-Friday, then 25 mcg extra on Monday and Tuesday. This keeps levels steady.

What If You’re Diagnosed During Pregnancy?

If you didn’t know you had hypothyroidism before getting pregnant, don’t panic. But act fast. TSH levels above 10 mIU/mL mean you need treatment immediately. Start at 1.6 mcg/kg/day and get tested within 2 weeks. Even mild cases (TSH 5-10) should be treated, especially if you have thyroid antibodies. Those antibodies mean your immune system is attacking your thyroid, and pregnancy can make it worse.

Studies show that women who start treatment early have outcomes nearly as good as those who were already on medication before pregnancy. Delaying treatment-even by a few weeks-can impact your baby’s development.

Real Stories, Real Challenges

One woman on Reddit shared that her OB told her to “wait and see” when she asked about increasing her dose at 6 weeks. Her TSH hit 4.2. She ended up needing another 25 mcg increase. She said the anxiety about her baby’s brain development was overwhelming.

Another woman, who followed her endocrinologist’s advice to increase her dose the day she got a positive test, kept her TSH perfectly in range. Her daughter scored in the 90th percentile for development at 18 months.

The difference? Timing. And advocacy. If your provider doesn’t mention thyroid testing, bring it up. Ask for a TSH test at your first visit. If they say it’s not necessary, ask for a referral to an endocrinologist. You’re not being pushy-you’re protecting your child.

What’s New in 2025?

Guidelines keep evolving. In 2023, the ATA reversed its stance and now recommends universal TSH screening for all pregnant women in the first trimester. That’s a big shift. It means every pregnant woman should get tested, not just those with symptoms or known thyroid issues.

AI tools are also entering the scene. A 2022 trial showed that an algorithm using pre-pregnancy TSH, weight, and antibody status predicted the right dose 28% more accurately than standard methods. These tools aren’t mainstream yet, but they’re coming.

Meanwhile, global access remains a problem. In low-income countries, only 22% have consistent access to levothyroxine. That’s why the WHO added it to its Essential Medicines List for maternal health in 2023. Thyroid care isn’t a luxury-it’s a basic right.

What About Breastfeeding?

Good news: levothyroxine is safe while breastfeeding. Only tiny amounts pass into breast milk, and they don’t affect the baby. In fact, your baby needs those hormones. Don’t stop or reduce your dose after birth. Your body still needs the same level of thyroid hormone as during pregnancy. You can return to your pre-pregnancy dose only after your TSH is checked at 6 weeks postpartum.

Just remember: your dose might still be higher than before pregnancy. Many women stay on their pregnancy dose long-term. That’s okay. Your thyroid may not bounce back to its pre-pregnancy state.

Final Checklist

If you’re pregnant or planning to be, here’s what to do:

1. Get your TSH tested before conception if you have a history of thyroid disease.
2. As soon as you confirm pregnancy, contact your doctor to increase your levothyroxine dose by 20-30%.
3. Get your first TSH check within 4 weeks of conception.
4. Test every 4 weeks until TSH stabilizes.
5. Take your pill on an empty stomach, 30-60 minutes before food.
6. Avoid calcium, iron, and prenatal vitamins within 4 hours of your dose.
7. After delivery, get your TSH checked at 6 weeks and adjust as needed.
8. Continue your medication while breastfeeding-no need to stop.

Thyroid health in pregnancy isn’t about fear. It’s about control. With the right dose and the right timing, you’re giving your baby the best possible start. You’re not just managing a hormone-you’re protecting a future.