Thyroid Medications in Pregnancy: How to Adjust Doses and Monitor TSH Levels

When you find out you're pregnant and you're already taking thyroid medication, the first question isn't just "Will I be okay?" - it's "How much more do I need to take?" and "When will I know if it's working?" The answer matters more than you think. Your baby’s brain is developing right now, and it depends on the thyroid hormone you’re giving it through your bloodstream. If your dose isn’t right, it can affect your child’s IQ, increase the chance of miscarriage, or lead to early birth. But here’s the good news: with the right adjustments and checks, you can keep your levels perfect - and your baby healthy.

Why Your Thyroid Dose Changes in Pregnancy

Your body doesn’t just change during pregnancy - it works harder. Even before you miss your period, your thyroid has to produce 50% more hormone to support both you and your growing baby. That’s because the placenta starts making hCG, a hormone that mimics TSH and pushes your thyroid into overdrive. By week 8, your baby’s own thyroid isn’t even active yet - so every bit of thyroid hormone it needs comes from you.

If you’re already on levothyroxine (the most common thyroid medication, sold as Synthroid® or generic LT4), your body will burn through your current dose faster. Studies show that 85% of women with pre-existing hypothyroidism need a higher dose during pregnancy. And most of that increase happens in the first trimester - often before you’ve even had your first prenatal visit.

How Much More Should You Take?

There’s no one-size-fits-all number, but there are clear, research-backed guidelines. If you were diagnosed with hypothyroidism before getting pregnant, most experts recommend increasing your daily dose by 20-30% as soon as you know you’re pregnant. That might mean adding one extra pill every other day, or increasing your daily dose by 12.5 to 25 mcg.

For example, if you were taking 75 mcg per day before pregnancy, you’d bump it up to 90-95 mcg right away. Some doctors follow the ACOG recommendation and give a flat 50 mcg increase on confirmation - especially if your TSH was already borderline high before conception. But if your TSH was over 10 mIU/mL before pregnancy, you might need an even bigger jump - up to 1.6 mcg per kg of body weight per day.

A 2021 NIH study tracked 280 pregnant women on levothyroxine. Their average dose went from 85.7 mcg before pregnancy to 100 mcg in the first trimester. That’s a 14.3 mcg increase - or about a 17% rise - just to keep up with pregnancy demands.

What TSH Levels Are Safe During Pregnancy?

TSH is your thyroid’s thermostat. In pregnancy, the target range changes with each trimester. The American Thyroid Association says TSH should be under 2.5 mIU/mL in the first trimester, and under 3.0 mIU/mL in the second and third. The Endocrine Society agrees - but adds that if you have thyroid antibodies (TPOAb), you should aim for under 2.5 throughout pregnancy.

Why so strict? Because your baby’s brain is forming neural connections between weeks 8 and 12 - and it can’t make its own thyroid hormone yet. A 2010 study found that women with TSH above 2.5 in the first trimester had a 69% higher risk of miscarriage. Another study showed that children born to mothers with uncontrolled TSH scored 7-10 points lower on IQ tests.

But here’s the catch: not all doctors agree. Some argue that pushing TSH below 2.5 might lead to overtreatment - giving you too much hormone, which can also be risky. Still, the majority of guidelines, including those from ACOG and the ATA, say: when in doubt, keep it low. And if you’re worried, ask for a TSH test at your first prenatal visit - don’t wait.

When and How Often Should You Get Tested?

Testing isn’t optional. It’s the only way to know if your dose is working. The ATA says to check TSH every 4 weeks after any dose change - until your levels stabilize. But here’s what actually happens in clinics: 68% of OB/GYNs don’t test TSH at the first prenatal visit for women with known thyroid disease.

That’s a problem. If your dose isn’t adjusted until week 12, your baby may have already missed critical brain development windows. The best practice? Get your TSH checked at 4-6 weeks gestation, then again at 8, 12, 16, 20, 24, 28, and 32-34 weeks. Some endocrinologists even test at 6 weeks and again at 10 weeks to catch the fastest rise in demand.

If your dose was adjusted at 6 weeks and your TSH is still above 2.5 at 8 weeks, you’ll likely need another increase. One patient on Reddit said her doctor waited until 10 weeks to adjust - her TSH was 4.2. She ended up needing another 25 mcg increase. That delay caused her serious anxiety. Don’t wait for symptoms. Don’t assume your old dose still works. Test early. Test often.

How to Take Levothyroxine Right

Taking your pill correctly is just as important as taking the right dose. Levothyroxine is absorbed poorly if you eat too soon after taking it. You need to wait 30 to 60 minutes before breakfast. Coffee, calcium supplements, iron pills, and even antacids can block absorption by 35-50%.

If you take prenatal vitamins with iron or calcium, take them at least 4 hours apart from your thyroid pill. Many women take their thyroid med in the morning and their prenatal at night - that’s the easiest fix.

Also, avoid taking extra doses on weekends only. Some doctors tell patients to take two extra pills per week to reach a 30% increase. But if you take them Saturday and Sunday, your TSH might spike on Monday morning because your body gets a big hit and then a long gap. Better to spread those extra doses across the week - say, Monday, Wednesday, and Friday - to keep levels steady.

What If You’re Diagnosed With Hypothyroidism During Pregnancy?

If you didn’t know you had hypothyroidism until you got pregnant, you’re not alone. About 2-3% of pregnant women are newly diagnosed. The approach is the same: start treatment right away.

If your TSH is 10 mIU/L or higher, start with 1.6 mcg per kg of body weight per day. If it’s between 5 and 10, start with 1.0 mcg per kg. Don’t wait for symptoms like fatigue or weight gain - those are common in pregnancy anyway. The goal is to get your TSH under 2.5 within 4 weeks.

A 2021 study showed that women who got their dose 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.

What About Breastfeeding?

Once your baby is born, you can keep taking levothyroxine. It passes into breast milk in tiny amounts - far too little to affect your baby. In fact, your baby needs that hormone. Your thyroid levels should return to pre-pregnancy levels after delivery, but don’t stop your medication without checking your TSH first. Many women need to reduce their dose back down after delivery - sometimes by 25-50%.

Wait 6-8 weeks after birth to retest. If your TSH is still high, you might need to stay on your pregnancy dose. If it’s too low, you can cut back. But don’t guess. Test.

What’s New in 2025?

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

AI tools are now being used to predict the exact dose you’ll need based on your pre-pregnancy TSH, weight, and antibody status. One 2022 trial showed AI-guided dosing improved TSH control by 28% compared to standard methods. The TRUST trial, expected to finish in late 2024, is testing whether personalized algorithms can reduce preterm birth even further.

And globally, levothyroxine is now on the WHO’s Essential Medicines List for maternal health. That means more low-income countries are starting to stock it - which could prevent thousands of cases of preventable developmental delays.

What to Do Right Now

If you’re pregnant and on thyroid medication:

• Call your doctor or endocrinologist today - don’t wait for your next appointment.
• Ask for a TSH test within the next week.
• If you’re not already on a dose increase, ask if you should start one now.
• Make sure you’re taking your pill on an empty stomach, 30-60 minutes before food.
• Keep your prenatal vitamins at least 4 hours apart from your thyroid pill.
• Track your TSH results in a notebook or app - the MyThyroid app has helped over 12,500 pregnant women stay on track.

What If Your Doctor Says No?

You’re not overreacting. You’re being smart. If your doctor says, "Wait and see," or "You’ll know if you feel bad," push back. Cite the guidelines: the American Thyroid Association, the Endocrine Society, and ACOG all agree - early testing and early dose increases save babies’ brains.

Bring a printed copy of the 2017 ATA guidelines or the 2020 ACOG bulletin. Say: "I want to follow the standard of care. Can we check my TSH this week?" If they still refuse, ask for a referral to an endocrinologist. Your baby’s development is worth it.

Thyroid medication in pregnancy isn’t about being "sick." It’s about being proactive. You’re not just taking a pill - you’re giving your child the best chance at a healthy brain, a strong start, and a full life.