Thyroid Medications in Pregnancy: Dose Adjustments and Monitoring

When you find out you're pregnant, your body starts changing in ways you can't always see. One of the most important but often overlooked changes is how your thyroid works. If you're already taking thyroid medication-usually levothyroxine-your dose isn't going to stay the same. In fact, most women need more medication almost immediately after conception. Skipping this adjustment can put your baby's brain development at risk.

Why Thyroid Medication Needs to Change During Pregnancy

Your thyroid gland makes hormones that control metabolism, energy, and, crucially, fetal brain development. Before 10 to 12 weeks, the baby can't make its own thyroid hormone. It relies entirely on what crosses the placenta from you. That’s why even a small drop in your thyroid hormone levels during the first trimester can affect your child’s IQ, attention, and learning abilities later in life.

Studies show that women with untreated or poorly managed hypothyroidism during pregnancy have a 69% higher risk of miscarriage and a 23% higher chance of preterm birth. On the flip side, keeping your TSH in range can improve your baby’s cognitive scores by 7 to 10 points compared to untreated cases. That’s not a small difference-it’s the difference between needing extra help in school and thriving without it.

How Much More Medication Do You Need?

Most women with pre-existing hypothyroidism need to increase their levothyroxine dose by 20% to 30% as soon as they confirm pregnancy. That might sound like a lot, but it’s not guesswork. Research from the National Institutes of Health found that women went from an average of 85.7 mcg per day before pregnancy to 100.0 mcg per day by the first trimester-an increase of nearly 15 mcg. That’s about one extra pill every few days.

There are two common ways doctors handle this:

• Take two extra doses per week (for example, take your pill on Monday and Thursday instead of just Monday).
• Or, if you're on a 100 mcg daily dose, increase it to 125 mcg right away.

The American College of Obstetricians and Gynecologists (ACOG) recommends a bigger jump-up to 50 mcg increase immediately upon confirmation. That’s because thyroid hormone demand spikes within days of conception, even before many women know they’re pregnant. Waiting until your first prenatal visit-often at 8 to 10 weeks-can be too late.

If you’re newly diagnosed with hypothyroidism during pregnancy, your starting dose depends on your TSH level:

• TSH ≥10 mIU/L: Start at 1.6 mcg per kg of body weight per day.
• TSH <10 mIU/L: Start at 1.0 mcg per kg per day.

For severe cases (TSH over 20), increases of 75 to 100 mcg per day may be needed. Mild cases (TSH between 5 and 10) usually need 25 to 50 mcg more.

What’s the Right TSH Target?

Not all guidelines agree on the exact TSH number you should aim for, but they all agree you need to stay below a certain threshold.

The American Thyroid Association (ATA) recommends keeping 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. Some experts, like Dr. Peter Laurberg, argue that pushing TSH too low might lead to overtreatment and unnecessary anxiety. But here’s the bottom line: if your TSH is above 2.5 in the first trimester, your risk of miscarriage goes up significantly.

Real-world data backs this up. One study found that women with TSH over 2.5 in early pregnancy had a 69% higher miscarriage rate than those who stayed under it. And if you’re positive for thyroid antibodies (TPOAb), the Endocrine Society says you should aim even tighter-TSH under 2.5 the whole time.

When and How Often Should You Get Tested?

Testing isn’t a one-time thing. You need to check your TSH frequently because your body’s demand keeps changing.

Here’s what the experts recommend:

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

Many OB/GYNs don’t check TSH at the first prenatal visit. A survey of 150 providers found that 68% skip this step. That’s dangerous. If your dose isn’t adjusted by week 6, you’re already behind.

Patients on Reddit and patient forums often report having to push for testing. One woman said her OB said, “Wait and see,” even though she was already 6 weeks along. Her TSH was 4.2. She had to get a second increase. That delay caused her months of stress.

How to Take Your Medication Correctly

Taking your pill wrong can make all the difference. Levothyroxine is absorbed poorly if you take it with food, coffee, calcium, or iron.

Follow these rules:

• Take it on an empty stomach, first thing in the morning.
• Wait 30 to 60 minutes before eating or drinking anything besides water.
• Avoid calcium supplements, prenatal vitamins with iron, or antacids for at least 4 hours after taking your pill.

These substances can block absorption by 35% to 50%. That means even if you’re taking the right dose, your body might not be getting it.

What About Breastfeeding?

Good news: levothyroxine is safe while breastfeeding. Only tiny amounts pass into breast milk, and studies show no effect on the baby’s thyroid function or development. You can keep your pregnancy dose or slightly reduce it after delivery-your doctor will guide you. Don’t stop or lower your dose on your own. Your thyroid needs time to reset after pregnancy, and stopping too soon can lead to postpartum hypothyroidism.

What’s New in 2025?

The field is moving fast. In 2023, the American Thyroid Association reversed its long-standing position and now recommends universal TSH screening for all pregnant women in the first trimester. That’s a big shift-from screening only high-risk women to testing everyone.

New tools are helping too. The ENDO trial in 2022 used artificial intelligence to predict individual dose needs based on pre-pregnancy TSH, weight, and antibody status. Women using AI-guided dosing had 28% better TSH control than those on standard protocols.

There’s also a mobile app called MyThyroid, used by over 12,500 pregnant women since 2019. Eighty-seven percent said it helped them stick to their schedule and reduce anxiety.

What If You Can’t Get Care?

Globally, access to levothyroxine is still unequal. In low-income countries, only 22% have consistent access to the medication. That contributes to 15% of preventable developmental delays in children. The World Health Organization added levothyroxine to its Essential Medicines List for maternal health in 2023, signaling that this isn’t just a personal health issue-it’s a public health priority.

What to Do Next

If you’re pregnant or planning to be:

1. Check your current TSH level before conception if possible.
2. As soon as you get a positive test, contact your endocrinologist or doctor. Don’t wait for your first OB appointment.
3. Ask for a TSH test within 4 weeks of confirmation.
4. Take your levothyroxine correctly-on an empty stomach, away from supplements.
5. Keep track of your doses and test dates. Use a calendar or app.

Don’t assume your doctor will know to check your thyroid. Many don’t. Advocate for yourself. Your baby’s brain development depends on it.