Yes, you can and should continue taking thyroid medication during pregnancy. For women with hypothyroidism (underactive thyroid), levothyroxine is considered safe and essential for both maternal health and fetal development. Stopping thyroid medication during pregnancy carries far greater risks than continuing it, and most women actually need a higher dose once they conceive.
Why Thyroid Medication Matters During Pregnancy
Your baby depends entirely on your thyroid hormones during the first trimester, before its own thyroid gland starts functioning around week 12. Even after that point, your hormone supply remains critical. When hypothyroidism goes untreated during pregnancy, the consequences are measurable: children born to mothers with untreated hypothyroidism scored an average of seven IQ points lower than children of mothers with normal thyroid function, and 19% had IQ scores below 85, compared to just 5% in the control group.
Beyond cognitive effects on the baby, untreated hypothyroidism raises the risk of miscarriage, preterm birth, preeclampsia, gestational hypertension, low birth weight, and premature rupture of membranes. A large meta-analysis found that women with subclinical hypothyroidism who took levothyroxine cut their risk of pregnancy loss by 45%, preterm birth by 37%, and gestational hypertension by 22% compared to those who went untreated.
Your Dose Will Likely Need to Increase
Pregnancy dramatically increases your body’s demand for thyroid hormone. On average, levothyroxine requirements jump about 47% during the first half of pregnancy, with the increase starting as early as week eight and plateauing around week 16. Because this ramp-up happens so quickly, many experts recommend increasing your dose by roughly 30% as soon as you get a positive pregnancy test, then fine-tuning from there based on blood work.
If you’re planning to become pregnant and already take levothyroxine, talk with your doctor beforehand so you have a plan in place. Waiting until your first prenatal appointment at 8 or 10 weeks may mean weeks of inadequate thyroid levels during a critical window for fetal brain development.
TSH Targets Are Tighter During Pregnancy
The thyroid hormone levels considered “normal” outside of pregnancy are too loose for pregnancy. Guidelines from the American Thyroid Association, the Endocrine Society, and the European Thyroid Association all recommend keeping TSH below 2.5 mIU/L during the first trimester and below 3.0 mIU/L during the second and third trimesters. These are notably lower than the upper limit of around 4.0 to 4.5 mIU/L used for the general population.
To stay within these ranges, thyroid function should be tested approximately every four weeks during the first half of pregnancy. After that, testing typically continues but may be less frequent if levels are stable.
Prenatal Vitamins Can Block Absorption
Here’s a practical detail that trips up many pregnant women: the iron and calcium in prenatal vitamins interfere with levothyroxine absorption. If you take them at the same time, your body may not absorb enough of the thyroid medication, effectively underdosing you even though you’re taking the right amount.
The fix is simple. Separate levothyroxine from any iron or calcium supplements, including prenatal vitamins, by at least four hours. The easiest approach is to take your thyroid medication first thing in the morning on an empty stomach, then take your prenatal vitamin with lunch or dinner.
Overactive Thyroid Requires a Different Approach
If you have hyperthyroidism (overactive thyroid) rather than hypothyroidism, the medication picture is more complicated but still manageable. Two main drugs treat overactive thyroid, and the preferred one changes depending on the trimester.
During the first trimester, propylthiouracil (PTU) is the preferred option because the alternative, methimazole, has been associated with rare but serious birth defects affecting the esophagus and nasal passages. After the first trimester, the recommendation flips: methimazole becomes the preferred choice because PTU carries a risk of liver toxicity with prolonged use. So many women with hyperthyroidism switch medications partway through pregnancy.
T3 and Desiccated Thyroid Are Not Recommended
If you currently take a combination T4/T3 medication, desiccated thyroid (sometimes called “natural” thyroid), or a standalone T3 supplement, pregnancy is the time to switch. Both the American Thyroid Association and the American Association of Clinical Endocrinologists recommend against using these preparations during pregnancy. The concern is that T3 supplementation can lower the mother’s T4 levels, and T4 is the form of thyroid hormone that crosses the placenta and supports fetal brain development. A mother can have a normal TSH reading while her T4 is actually too low for the baby’s needs.
Levothyroxine (pure T4) is the standard and safest option during pregnancy. Your body and your baby’s body convert T4 into the active T3 form as needed.
What Happens After Delivery
Once you deliver, your thyroid hormone needs drop back down. How quickly and how much your dose changes depends on why you started medication in the first place.
- Overt hypothyroidism diagnosed during pregnancy: the dose is typically reduced to about two-thirds of the final pregnancy dose immediately after delivery.
- Subclinical hypothyroidism with positive thyroid antibodies: the dose is usually cut to half of the final pregnancy dose.
- Subclinical hypothyroidism without thyroid antibodies: women on low doses (25 mcg or less) often stop the medication entirely, while those on higher doses are reduced to about half.
- Hypothyroidism that existed before pregnancy: you’ll generally return to your pre-pregnancy dose, though this should be confirmed with blood work.
Regardless of the scenario, thyroid levels are typically rechecked about six weeks after delivery to make sure the adjusted dose is correct. Thyroid function can also shift unpredictably in the postpartum period due to postpartum thyroiditis, so continued monitoring matters even if your levels were stable throughout pregnancy.