Thyroid Medications in Pregnancy: Dose Adjustments and Monitoring

Why Thyroid Medication Matters in Pregnancy

When you’re pregnant, your body doesn’t just need more food or rest-it needs more thyroid hormone. That’s because your baby relies entirely on your thyroid hormones during the first 10 to 12 weeks of development, before its own thyroid even starts working. If your thyroid isn’t producing enough, or if your medication dose isn’t adjusted, your baby’s brain development could be affected. Studies show that untreated or poorly managed hypothyroidism during pregnancy can lower a child’s IQ by 7 to 10 points and increase the risk of miscarriage by up to 69%. The good news? With the right medication and monitoring, these risks drop dramatically. Levothyroxine, the standard treatment, is safe, well-studied, and classified as Pregnancy Category A by the FDA-meaning no proven risk to the fetus.

How Much More Medication Do You Need?

If you’re already taking levothyroxine before pregnancy, you’re likely going to need more. On average, women increase their dose by 20% to 30% as soon as they find out they’re pregnant. Some guidelines, like those from the American College of Obstetricians and Gynecologists (ACOG), suggest an immediate 50 mcg increase upon confirmation. Others, like the American Thyroid Association (ATA), recommend a 20-30% increase, which many patients achieve by taking two extra doses per week. For example, if you were taking 75 mcg daily before pregnancy, you’d move to about 90-95 mcg daily once pregnant.

But it’s not just about doubling up. If you’re newly diagnosed during pregnancy, dosing depends on your TSH level. If your TSH is above 10 mIU/L, start with 1.6 mcg per kilogram of body weight per day. If it’s between 5 and 10, start with 1.0 mcg/kg/day. A 2021 NIH study tracking 280 pregnant women found that their average levothyroxine dose jumped from 85.7 mcg before pregnancy to 100 mcg by the first trimester-that’s a 16.7% increase in just weeks.

When to Adjust Your Dose

Don’t wait for your first prenatal visit. Thyroid hormone demand spikes almost immediately after conception, often before you even know you’re pregnant. Experts agree: the best time to increase your dose is the day you get a positive pregnancy test. Delaying adjustment-even by a few weeks-can put your baby at risk. One patient on Reddit shared that her OB told her to "wait and see," and her TSH stayed at 4.2 at 8 weeks. She ended up needing another 25 mcg increase. That delay caused her months of anxiety. On the flip side, another woman increased her dose right away, kept her TSH in range, and her daughter scored in the 90th percentile for development at 18 months.

How Often Should You Get Tested?

Testing isn’t a one-time thing. You need to check your TSH every 4 weeks until your dose stabilizes, then at least once per trimester. The ATA recommends testing at 4-6 weeks, then every 4-6 weeks until 20 weeks, and again at 24-28 weeks and 32-34 weeks. Why so often? Because your body’s need for thyroid hormone keeps rising. By week 16, most women are already on their highest dose. A 2013 study found that 75% of women with pre-existing hypothyroidism needed a dose increase in the first trimester alone.

Target TSH levels vary slightly between guidelines. The ATA says keep TSH ≤ 2.5 mIU/L throughout pregnancy. The Endocrine Society says ≤ 2.5 in the first trimester, and up to 3.0 in the second and third. But here’s the key: if your TSH is above 2.5 in the first trimester, your risk of miscarriage goes up by nearly 70%. That’s why many endocrinologists aim for the stricter target, especially if you have thyroid antibodies.

How to Take Your Medication Right

It’s not just about the dose-it’s about how you take it. Levothyroxine works best when taken on an empty stomach, at least 30 to 60 minutes before eating. If you take it with coffee, calcium, iron, or prenatal vitamins containing those minerals, your body absorbs 35% to 50% less. That means your TSH could creep up even if you’re taking the "right" dose.

Some patients try to make up for missed doses by taking extra pills on weekends. But that can backfire. Taking two extra doses on Saturday and Sunday might cause a spike in hormone levels Monday morning, then a drop by Tuesday. Instead, spread the extra doses across the week-add 3.5 mcg extra per day, every day, rather than 25 mcg on two days. This keeps your levels steady.

What Happens If You Don’t Adjust?

Ignoring dose changes isn’t just risky-it’s common. A survey of 150 OB/GYNs found that 68% don’t routinely check TSH at the first prenatal visit for women with known hypothyroidism. That’s a major gap. Without timely adjustments, you’re more likely to face preterm birth, low birth weight, or even preeclampsia. A 2021 study showed women whose doses were adjusted within four weeks of pregnancy confirmation had 23% fewer preterm births than those who waited.

And it’s not just about the baby. Untreated hypothyroidism can lead to fatigue, depression, muscle cramps, and high cholesterol in the mother. It can also interfere with labor and postpartum recovery. The stakes are high, but the solution is simple: test early, adjust early, and test again.

What’s New in 2025?

Thyroid care in pregnancy is getting smarter. In 2023, the ATA reversed its long-standing stance and now recommends universal TSH screening for all pregnant women in the first trimester-not just those with symptoms or risk factors. That’s a big shift. Meanwhile, AI tools are starting to predict your ideal dose based on your pre-pregnancy TSH, weight, and thyroid antibody status. The 2022 ENDO trial showed these tools improved TSH control by 28% compared to standard dosing.

There’s also growing global recognition. The WHO added levothyroxine to its Essential Medicines List for maternal health in 2023. But access remains unequal: in low-income countries, only 22% have consistent access to the medication, contributing to 15% of preventable developmental delays in those regions.

What You Can Do Today

• If you’re planning pregnancy or just found out you’re pregnant and take levothyroxine, contact your doctor immediately. Don’t wait.
• Ask for a TSH test right away-even if you feel fine.
• Take your medication on an empty stomach, 30-60 minutes before food.
• Avoid calcium, iron, and prenatal vitamins within 4 hours of taking your pill.
• Track your doses and test dates using a free app like MyThyroid, used by over 12,500 pregnant women.
• Insist on follow-up tests every 4 weeks until your dose is stable.

Thyroid health in pregnancy isn’t optional. It’s essential. With the right care, you can protect your baby’s brain development and have a healthy pregnancy. You don’t need to guess. You don’t need to wait. You just need to act-and make sure your care team does too.