Thyroid Medications in Pregnancy: Dose Adjustments and Monitoring

When you find out you're pregnant, your body changes 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 likely needs to change. Right away. Not next month. Not at your next appointment. Levothyroxine is the only thyroid medication recommended during pregnancy, and getting the dose wrong can affect your baby’s brain development.

Why Thyroid Hormone Matters in Pregnancy

Your baby doesn’t make its own thyroid hormone until about 12 weeks into pregnancy. Before that, it relies entirely on you. Thyroid hormone isn’t just about energy or weight-it’s critical for building your baby’s brain and nervous system. Studies show that untreated or poorly managed hypothyroidism during pregnancy can lower a child’s IQ by 7 to 10 points. It also increases the risk of miscarriage, preterm birth, and preeclampsia.

Here’s the hard truth: 85% of women with pre-existing hypothyroidism need a higher dose of levothyroxine during pregnancy. And 75% of those increases are needed by the end of the first trimester. That means if you’re taking the same dose you were before pregnancy, you’re likely undermedicated-sometimes dangerously so.

How Much More Do You Need?

There’s no one-size-fits-all answer, but guidelines give us clear starting points.

If you already have hypothyroidism and just found out you’re pregnant, most experts recommend increasing your dose by 20% to 30% as soon as possible. That’s about 12.5 to 25 micrograms more per day. For someone taking 100 mcg daily, that means jumping to 120-125 mcg. Some doctors, especially those following ACOG guidelines, recommend an immediate 50 mcg increase, regardless of current dose.

For women newly diagnosed with hypothyroidism during pregnancy, the starting dose depends on how high their TSH is:

• If TSH is 10 mIU/L or higher: start at 1.6 mcg per kg of body weight per day
• If TSH is under 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 went from 85.7 mcg before pregnancy to 100.0 mcg in the first trimester-an increase of 16.7%. That’s not small. That’s the difference between a healthy pregnancy and a risky one.

When to Adjust the Dose

Don’t wait for your first prenatal visit. Thyroid hormone demand spikes within days of conception. By the time you miss your period, your body already needs more. Waiting until week 6 or 8 to adjust your dose is too late for your baby’s brain.

Many women report having to push back against their OB-GYNs who say, “Let’s wait and see.” But guidelines from the American Thyroid Association are clear: increase the dose immediately upon confirmation of pregnancy. If you’re unsure, ask for a copy of the 2017 ATA guidelines. Most doctors have access to them.

One patient shared on a support forum: “My doctor waited until 8 weeks. My TSH was 4.2. I had to get another increase. I was terrified the whole time.” That fear is real-and avoidable.

How Often Should You Get Tested?

Testing TSH every 4 weeks is the standard. But timing matters.

Here’s a practical schedule:

1. Test TSH as soon as pregnancy is confirmed
2. Recheck every 4 weeks until TSH stabilizes
3. After stabilization, test again at 24-28 weeks and 32-34 weeks

Some doctors skip the first test entirely. A 2019 survey found that 68% of OB-GYNs don’t check TSH at the first prenatal visit for women with known hypothyroidism. That’s a gap in care. Don’t assume it’s being done. Ask for it.

What TSH Level Is Target?

This is where opinions split. The American Thyroid Association says TSH should stay at or below 2.5 mIU/L throughout pregnancy. The Endocrine Society agrees for the first trimester but allows up to 3.0 mIU/L in the second and third. Other experts argue that pushing TSH below 2.5 might lead to overtreatment.

But here’s what the data says: women with TSH above 2.5 in the first trimester have a 69% higher risk of miscarriage. That’s not a small risk. It’s a reason to aim for the tighter target.

There’s no perfect number, but if your TSH is over 2.5, especially in early pregnancy, your doctor should consider increasing your dose-even if you feel fine. Symptoms don’t always match your lab results.

How to Take Levothyroxine Correctly

Taking your pill right matters just as much as how much you take.

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

These supplements can block up to half the absorption of your medication. If your prenatal vitamin has iron or calcium, take it at dinnertime, not with your thyroid pill.

Some patients try to make up for missed doses by taking extra pills on weekends. That’s risky. Taking two extra doses on Saturday and Sunday can cause your TSH to spike on Monday. Better to spread the extra 12.5-25 mcg over the week-take an extra 1/4 tablet every other day.

What About Breastfeeding?

Good news: levothyroxine is safe during 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 return to your pre-pregnancy dose after delivery. Your doctor will likely check your TSH 6 weeks postpartum to make sure you’re back on the right track.

Real Challenges and How to Overcome Them

Even with clear guidelines, things go wrong. Patients report delays, dismissals, and confusion. Here’s how to protect yourself:

• Keep a copy of your pre-pregnancy TSH and dose
• Write down your target TSH range (2.5 or below)
• Ask for your labs to be sent to you-don’t wait for a call
• If your doctor says “wait,” ask: “What’s the risk if I don’t adjust now?”

Technology is helping. Apps like MyThyroid have been used by over 12,500 pregnant women and helped 87% improve their medication adherence. Electronic health records like Epic now have alerts that pop up when a pregnant patient is on thyroid medication-prompting doctors to check TSH.

What’s Changing in 2026?

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

AI tools are being tested to predict the exact dose a woman needs based on her weight, TSH, and thyroid antibodies. One 2022 trial showed AI-guided dosing improved TSH control by 28% compared to standard methods. Clinical trials are ongoing, and precision dosing could become standard within the next few years.

But for now, the rules are simple: increase your dose early. Test often. Take your pill right. Advocate for yourself. Your baby’s brain is counting on it.

What Happens If You Don’t Adjust?

The risks aren’t theoretical. 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. Delayed adjustments also correlate with lower cognitive scores in children at age 2.

And it’s not just about IQ. Babies born to mothers with uncontrolled hypothyroidism are more likely to have low birth weight, respiratory problems, and developmental delays. These aren’t rare outcomes-they’re preventable ones.

What to Do Next

If you’re pregnant and on thyroid medication:

• Call your endocrinologist or doctor today and ask: “Should I increase my dose now?”
• Request a TSH test within the next week
• Review your dosing schedule with your pharmacist
• Download the MyThyroid app to track your doses and labs
• Keep a printed copy of the 2017 ATA guidelines to show your provider if needed

Thyroid health in pregnancy isn’t optional. It’s essential. And with the right steps, you can protect your baby’s future-and your own peace of mind.