Thyroid Medications in Pregnancy: Dose Adjustments and Monitoring

When you're pregnant and taking thyroid medication, your body isn't just changing-it's demanding more. The thyroid hormone your baby needs for brain development comes from you, and your dose isn't the same as it was before pregnancy. Many women don't realize they need more medication until it's too late. That delay can affect your baby's future learning, behavior, and even IQ. The good news? With the right adjustments and regular checks, you can keep both you and your baby healthy.

Why Thyroid Medication Changes During Pregnancy

Your thyroid gland works harder during pregnancy. It has to make about 50% more hormone to support both you and your growing baby. That’s because your baby can’t make its own thyroid hormone until around 12 weeks. Before that, it relies entirely on what you’re giving it through the placenta. If your thyroid levels drop-even slightly-your baby’s brain development can be affected.

The main medication used is levothyroxine, sold under brands like Synthroid®. It’s safe, effective, and has been used for decades in pregnant women. But here’s the key: the dose you were taking before you got pregnant is almost never enough once you’re pregnant. Studies show that 85% of women with pre-existing hypothyroidism need a higher dose during pregnancy. And 75% of those increases are needed by the time you’re 8 weeks along.

How Much More Medication Do You Need?

There’s no one-size-fits-all answer, but there are clear guidelines. If you already have hypothyroidism and find out you’re pregnant, most experts recommend increasing your daily dose by 20% to 30% right away. That’s about 12.5 to 25 mcg extra per day. Some doctors, especially those following ACOG guidelines, will jump straight to a 50 mcg increase.

For example, if you were taking 75 mcg per day before pregnancy, you might move to 90-100 mcg as soon as you get a positive test. You don’t need to wait for your first OB visit. The earlier you adjust, the better for your baby.

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

• If your TSH is 10 mIU/L or higher: start at 1.6 mcg per kg of body weight per day.
• If your TSH is below 10 mIU/L: start at 1.0 mcg per kg per day.

A 2021 NIH study tracked 280 pregnant women and found their average levothyroxine dose increased from 85.7 mcg before pregnancy to 100.0 mcg in the first trimester. That’s a 16.7% rise-just from conception to week 12.

When to Adjust Your Dose

Timing matters more than you think. Thyroid hormone demand spikes immediately after conception, often before a woman even knows she’s pregnant. That’s why experts say: don’t wait for your first prenatal appointment. If you’re trying to conceive or think you might be pregnant, talk to your doctor about increasing your dose now.

Many women wait until their first OB visit at 8-10 weeks. By then, their baby has already been developing without enough thyroid hormone for 4-6 weeks. That window is critical. The first 10-12 weeks are when the baby’s brain is forming its basic structure. Missing the mark here can lead to lower IQ scores later.

A 2021 study found that women whose doses were adjusted within 4 weeks of pregnancy confirmation had 23% fewer preterm births than those who waited longer. That’s not a small difference-it’s life-changing.

How Often Should You Get Tested?

Checking your TSH (thyroid-stimulating hormone) isn’t optional. It’s essential. The American Thyroid Association says to test your TSH every 4 weeks until your levels stabilize. That usually means testing at:

• 4-6 weeks gestation
• 8-10 weeks
• 12-16 weeks
• 24-28 weeks
• 32-34 weeks

Some doctors skip the early tests. A survey of 150 OB/GYNs found that 68% didn’t check TSH at the first prenatal visit for women with known hypothyroidism. That’s a problem. You can’t adjust what you don’t measure.

And don’t assume your old TSH target still works. During pregnancy, the goal changes:

• First trimester: TSH should be ≤ 2.5 mIU/mL
• Second trimester: TSH ≤ 3.0 mIU/mL
• Third trimester: TSH ≤ 3.0 mIU/mL

Some guidelines, like the ATA’s, recommend keeping TSH ≤ 2.5 throughout pregnancy. Others allow a bit more leeway later on. But here’s what all experts agree on: if your TSH is above 2.5 in the first trimester, your risk of miscarriage goes up by 69%.

What Happens If You Don’t Adjust?

Skipping dose adjustments isn’t just risky-it’s preventable. Untreated or under-treated hypothyroidism during pregnancy is linked to:

• Higher chance of miscarriage
• Preterm birth
• Preeclampsia
• Low birth weight
• Lower IQ in children-studies show up to a 7-10 point drop compared to babies of mothers with well-controlled thyroid levels

One patient on a thyroid forum shared: “My doctor waited until 8 weeks to increase my dose. My TSH was 4.2. I was terrified my baby wouldn’t be okay.” That fear isn’t irrational. Research backs it up.

On the flip side, another woman said: “I increased my dose the day I got a positive test. My TSH stayed perfect. My daughter is now in the 90th percentile for development at 18 months.” That’s the difference good management makes.

How to Take Your Medication Correctly

Taking levothyroxine right matters just as much as taking the right dose. Here’s what you need to know:

• Take it on an empty stomach-first thing in the morning, 30 to 60 minutes before eating.
• Avoid calcium, iron, or prenatal vitamins within 4 hours. These block absorption by 35-50%.
• Don’t switch brands without checking with your doctor. Even though generic levothyroxine is approved, small differences in absorption can throw off your levels.
• If you miss a dose, take it as soon as you remember. If it’s close to your next dose, skip the missed one. Don’t double up.

Some women try to increase their weekly dose by taking extra pills on weekends. That can cause TSH spikes on Monday mornings. Better to spread the extra dose evenly across the week-like adding 3.5 mcg extra per day instead of 25 mcg on Saturday.

What About Breastfeeding?

Good news: levothyroxine is safe while breastfeeding. Very little of the medication passes into breast milk-far less than what your baby would make naturally. You don’t need to stop or reduce your dose after delivery. In fact, many women go back to their pre-pregnancy dose after giving birth, but only after checking TSH levels around 6 weeks postpartum.

Your baby’s thyroid function is checked at birth with the newborn screening test. That’s separate from your own thyroid levels. Don’t assume your baby’s test result means yours are fine. You still need your own follow-up.

What’s New in 2025?

The field is moving fast. In 2023, the American Thyroid Association reversed its stance and now recommends universal TSH screening for all pregnant women in early pregnancy-not just those with symptoms or a history. That’s a big shift.

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

Hospitals are catching up. Systems like Epic now have built-in alerts that pop up when a pregnant patient is on levothyroxine, reminding doctors to check TSH and adjust dose.

Still, access isn’t equal. 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 just a personal issue-it’s a global health priority.

What You Can Do Today

If you’re pregnant and on thyroid medication:

• Call your doctor right away-even if you just suspect you’re pregnant.
• Ask for a TSH test within the first 6 weeks.
• Request a dose increase of 20-30% if you have pre-existing hypothyroidism.
• Take your medication correctly: empty stomach, no calcium or iron for 4 hours.
• Track your doses and test dates. Use apps like MyThyroid if they help.
• Don’t wait for your OB to bring it up. Be your own advocate.

If you’re planning pregnancy:

• Get your TSH checked before you conceive.
• Make sure it’s below 2.5 mIU/mL.
• Work with your endocrinologist to adjust your dose ahead of time.

Your thyroid isn’t just a gland-it’s your baby’s first brain builder. Getting it right isn’t optional. It’s one of the most powerful things you can do for your child’s future.