Thyroid Medication During Pregnancy: What You Need to Know

When you're pregnant and have hypothyroidism, a condition where the thyroid doesn't make enough hormone. Also known as underactive thyroid, it can affect everything from your energy to your baby’s brain development. Left untreated, low thyroid hormone levels during pregnancy raise the risk of preterm birth, low birth weight, and even learning delays in children. That’s why levothyroxine, the most common thyroid replacement drug. Also known as Synthroid or Tirosint, it’s one of the few medications doctors strongly recommend continuing—or starting—during pregnancy. It’s safe, well-studied, and critical for your baby’s nervous system, especially in the first trimester when the fetus relies entirely on your thyroid hormone.

Your body’s demand for thyroid hormone jumps by 30% to 50% early in pregnancy, so many women need a higher dose within the first 8 weeks. If you’re already on levothyroxine before getting pregnant, don’t wait—talk to your doctor right away. Waiting until your first prenatal visit could mean your baby misses out on vital hormone support during key development stages. Your doctor will check your TSH levels, the hormone that tells your thyroid when to produce more. Also known as thyroid-stimulating hormone, it’s the main marker used to adjust your dose. Target TSH levels during pregnancy are tighter than normal: under 2.5 mIU/L in the first trimester, and under 3.0 mIU/L later on. Many women need blood tests every 4 to 6 weeks until their levels stabilize.

Not all thyroid meds are safe in pregnancy. Drugs like methimazole or propylthiouracil are used for overactive thyroid (hyperthyroidism), but they carry different risks and require careful monitoring. If you’re on one of these, your doctor may switch you to a safer option as soon as possible. And while levothyroxine is safe, it doesn’t mix well with certain supplements—iron, calcium, and prenatal vitamins can block its absorption. Take it on an empty stomach, at least 30 to 60 minutes before eating or taking anything else.

After delivery, your thyroid needs often drop back down. But don’t stop your meds without checking in. Many women need to return to their pre-pregnancy dose, and skipping a follow-up test can lead to fatigue, weight gain, or postpartum depression. If you’re breastfeeding, levothyroxine is perfectly safe—only tiny amounts pass into milk, and no harm has been shown to babies.

What you’ll find below are real, practical guides from doctors and patients who’ve been through this. From how to track your TSH at home to what to do when your dose feels off, these posts cut through the noise. You’ll see how levothyroxine fits into daily life during pregnancy, why some women need extra monitoring, and what signs you should never ignore. No fluff. Just what works.