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. Their own thyroid doesn’t start working until then. If your thyroid levels are too low, it can affect your baby’s brain development, increase the risk of miscarriage, or lead to preterm birth. The good news? This is one of the most manageable conditions in pregnancy-if you know what to do.

Levothyroxine (LT4), the synthetic form of thyroid hormone, is the only medication recommended for hypothyroidism during pregnancy. Brands like Synthroid® are commonly used, but the generic version works just as well if taken consistently. What matters isn’t the brand-it’s the dose and how often you get checked.

How Much More Medication Do You Need?

If you were already on levothyroxine before getting pregnant, you’ll likely need more. On average, women increase their dose by 20% to 30% as soon as they find out they’re pregnant. That’s not a suggestion-it’s a medical necessity. The thyroid hormone demand starts rising the moment conception happens, often before a woman even knows she’s pregnant.

A study of 280 women showed their average dose jumped from 85.7 mcg before pregnancy to 100.0 mcg in the first trimester. That’s a 14.3 mcg increase in just a few weeks. For some, that means adding an extra 12.5 to 25 mcg daily. Others may need to increase by 50 mcg right away, especially if their TSH was already high before conception.

Here’s what experts recommend based on your starting TSH level:

• If your TSH is over 10 mIU/L, start at 1.6 mcg per kg of body weight per day.
• If your TSH is between 5 and 10 mIU/L, increase your current dose by 25 to 50 mcg per day.
• If your TSH is over 20 mIU/L, you may need a jump of 75 to 100 mcg per day.

Some doctors suggest taking two extra doses per week instead of increasing daily. For example, if you normally take 100 mcg Monday through Sunday, you’d take 125 mcg on Tuesday and Friday. But this can cause spikes and dips in hormone levels. A better approach is to spread the extra dose evenly across the week-like adding 5 mcg to your daily dose.

When to Adjust Your Dose

Don’t wait for your first prenatal visit. If you’re trying to conceive or think you might be pregnant, talk to your doctor about increasing your dose immediately. Delaying adjustment by even a few weeks can hurt fetal brain development. One study found that women who got their dose adjusted within four weeks of confirmation had 23% fewer preterm births than those who waited.

Many OB/GYNs don’t check thyroid levels at the first appointment. A survey of 150 OB/GYNs found that 68% didn’t routinely test TSH in pregnant women with known hypothyroidism. That means you might need to speak up. Bring your pre-pregnancy dose with you. Ask for a TSH test right away. Don’t let anyone tell you to “wait and see.”

What TSH Levels Are Safe During Pregnancy?

Target TSH levels change with each trimester. The American Thyroid Association (ATA) recommends keeping TSH below 2.5 mIU/L throughout pregnancy. The Endocrine Society allows up to 3.0 mIU/L in the second and third trimesters. But here’s the key point: the stricter the target, the better the outcomes.

Women with TSH above 2.5 mIU/L in the first trimester have a 69% higher risk of miscarriage. That’s not a small number. Even if you feel fine, your baby isn’t. Studies show that children born to mothers with untreated or poorly controlled hypothyroidism score 7 to 10 points lower on IQ tests than those whose mothers had well-managed thyroid levels.

There’s debate among experts. Some argue that TSH up to 4.0 mIU/L is fine in later pregnancy, citing data from Denmark. But most major guidelines, including ACOG and ATA, still push for the tighter target. Why? Because the brain is developing fast in the first trimester-and that’s when you need the most hormone.

How Often Should You Get Tested?

Testing isn’t optional-it’s non-negotiable. You need a TSH blood test every 4 weeks until your dose is stable. That usually means checking at:

1. 4 to 6 weeks gestation
2. 8 to 10 weeks
3. 16 weeks
4. 20 to 24 weeks
5. 28 to 32 weeks

After that, you can space tests out to every 6 weeks unless your levels start to drift. But if you’re on a new dose, test again in 4 weeks. Don’t assume your dose is right just because you felt fine last time.

Some clinics now use electronic health records that automatically flag pregnant patients and remind doctors to check thyroid levels. But if yours doesn’t, you’ll have to be the one to ask.

How to Take Levothyroxine Correctly

It’s not just about the dose-it’s about how you take it. Levothyroxine is absorbed poorly if you eat too soon after taking it. Take it on an empty stomach, at least 30 to 60 minutes before breakfast. Coffee, calcium, iron, and even soy can block absorption by up to 50%.

If you take prenatal vitamins with iron or calcium, take them at least 4 hours apart from your thyroid pill. Same with antacids, fiber supplements, or multivitamins. Many women don’t realize this and wonder why their TSH keeps creeping up.

Consistency matters too. Take it at the same time every day. If you forget, take it as soon as you remember-unless it’s close to your next dose. Don’t double up.

What About Breastfeeding?

Good news: levothyroxine is safe while breastfeeding. Only tiny amounts pass into breast milk-far less than what a newborn naturally produces. You don’t need to adjust your dose after delivery. In fact, many women go back to their pre-pregnancy dose after giving birth. But wait until your TSH is checked 6 to 8 weeks postpartum before making any changes.

Some women feel exhausted after delivery and assume it’s “baby fatigue.” But if your TSH is high, it could be your thyroid. Don’t ignore it. Postpartum thyroiditis is common, and untreated, it can lead to long-term hypothyroidism.

Real Stories, Real Challenges

One patient on Reddit shared how her OB told her to “wait and see” when she asked for a dose increase at 6 weeks. Her TSH hit 4.2 by 8 weeks. She had to push hard for another increase. She said the anxiety over her baby’s development was worse than the pregnancy itself.

Another woman, who increased 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. That’s not luck-it’s science.

Doctors aren’t always up to date. Only 78% of OB/GYNs follow current thyroid guidelines. That means you might be your own best advocate. Bring printouts from the American Thyroid Association. Ask for specific tests. Don’t settle for vague answers.

What’s New in 2026?

AI is starting to help. 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. By 2026, tools like this are becoming more common in clinics.

Also, the WHO now lists levothyroxine as a priority medicine for maternal health. That means more countries are making it available in low-income areas. But globally, 15% of preventable developmental delays still happen because women can’t get consistent access to this simple, life-changing drug.

Universal screening for TSH in early pregnancy is now recommended by the ATA-something they didn’t suggest just a few years ago. That’s a big shift. It means more women will be caught early, before damage is done.

Bottom Line: Do This Now

If you’re pregnant and on thyroid medication:

• Call your doctor today-don’t wait for your next appointment.
• Ask for a TSH test immediately.
• Increase your dose by 20-30% if you haven’t already.
• Take your pill correctly: empty stomach, 30-60 minutes before food, 4 hours away from iron or calcium.
• Test every 4 weeks until your levels are stable.
• Don’t stop-even if you feel fine.

Thyroid disease in pregnancy is not a crisis. It’s a manageable condition-with the right steps. The difference between a healthy baby and a child with developmental delays often comes down to one simple thing: timely dose adjustment. Don’t let a delay cost your child their potential.