High TSH during pregnancy is treated with levothyroxine, a synthetic thyroid hormone that replaces what your body isn’t making enough of. The target is generally a TSH at or below 2.5 mIU/L in the first trimester and below 3 mIU/L in the second and third trimesters. How aggressively your provider treats depends on how high your TSH is, which trimester you’re in, and whether you have thyroid antibodies.
Why TSH Rises During Pregnancy
Your thyroid works harder during pregnancy than at almost any other point in your life. The developing baby depends entirely on your thyroid hormones for the first 12 to 14 weeks, before its own thyroid gland starts functioning. Your body needs to produce roughly 50% more thyroid hormone than usual to meet this demand. When the thyroid can’t keep up, TSH rises as your brain signals for more hormone production.
Some women discover this for the first time through routine prenatal bloodwork. Others already have a hypothyroidism diagnosis and find that their previously stable medication dose is no longer sufficient. Either way, the priority is the same: bring TSH into the target range quickly, especially during the first trimester when fetal brain development is most dependent on maternal thyroid hormone.
What Untreated High TSH Can Do
Leaving high TSH untreated during pregnancy carries real risks for both you and the baby. Maternal hypothyroidism is linked to miscarriage, preterm delivery (before 37 weeks), preeclampsia, gestational diabetes, and placental abruption. The risks aren’t limited to severe cases. Preterm birth is more common even in mothers with subclinical hypothyroidism, where TSH is only mildly elevated.
For the baby, the consequences can extend well beyond birth. Insufficient thyroid hormone during pregnancy is associated with lower IQ scores and delays in attention, language, reading, motor skills, and visual-spatial abilities. These effects on brain development are the main reason guidelines emphasize early and consistent treatment throughout pregnancy.
When Treatment Is Recommended
If your TSH is above 10 mIU/L at any point during pregnancy, treatment with levothyroxine is strongly recommended regardless of other factors. For milder elevations, the decision depends on timing and antibody status.
Subclinical hypothyroidism in pregnancy means your TSH is above the trimester-specific upper limit but below 10 mIU/L. If this is found in the first trimester, your provider will typically start a low dose of levothyroxine or retest in one to three weeks to confirm the diagnosis before beginning treatment. The first trimester is the most critical window because of the baby’s reliance on your thyroid hormones.
If subclinical hypothyroidism is first detected in the second or third trimester, the approach is more conservative. Treatment at that stage hasn’t been shown to change pregnancy outcomes, so monitoring with thyroid function tests every four to six weeks through 20 weeks, then again around 28 weeks, is often sufficient.
The Role of TPO Antibodies
Thyroid peroxidase (TPO) antibodies are markers of autoimmune thyroid disease, the most common cause of hypothyroidism. If you test positive for these antibodies, your provider is more likely to start treatment even when your TSH is only mildly elevated. The antibodies signal that your thyroid is under immune attack and may struggle to meet the increasing demands of pregnancy. Guidelines specifically recommend treating TPO-positive women to reach a preconception TSH of 2.5 mIU/L or lower.
What Treatment Looks Like
Levothyroxine is the only medication used to treat hypothyroidism during pregnancy. It’s a synthetic version of the T4 hormone your thyroid naturally produces, and it has a long safety record in pregnancy.
If you were already taking levothyroxine before becoming pregnant, the standard recommendation is to increase your dose by about 30% as soon as pregnancy is confirmed. A commonly cited approach is to take two extra doses per week at your current strength. Research published in the New England Journal of Medicine found that levothyroxine requirements increase by an average of 47% during the first half of pregnancy, with most of the increase happening by week 16, after which the dose typically stabilizes.
If you’re newly diagnosed, your provider will start you on a dose based on your TSH level and body weight, then adjust based on follow-up bloodwork. TSH should be checked approximately every four weeks during the first half of pregnancy, with additional testing around 28 weeks.
Getting the Most From Your Medication
How you take levothyroxine matters as much as the dose. The medication is best absorbed on an empty stomach, ideally 30 to 60 minutes before eating. The more important rule during pregnancy: separate levothyroxine from your prenatal vitamin by at least four hours. Prenatal vitamins contain iron and calcium, both of which bind to levothyroxine in the gut and dramatically reduce how much your body absorbs.
Many women find it easiest to take levothyroxine first thing in the morning and their prenatal vitamin at lunch or bedtime. If you’ve been taking both together and your TSH isn’t coming down, this timing issue could be the reason.
Iodine and Nutrition
Iodine is the raw material your thyroid needs to produce hormones. The recommended daily intake jumps from 150 micrograms before pregnancy to 220 micrograms during pregnancy and 290 micrograms while breastfeeding. The American Thyroid Association and the American Academy of Pediatrics recommend that pregnant women take a daily prenatal supplement containing 150 micrograms of iodine to help bridge the gap between dietary intake and the increased demand.
Not all prenatal vitamins contain iodine, so check your label. Dietary sources include iodized salt, dairy products, eggs, and seafood. Iodine supplementation supports your thyroid function but does not replace levothyroxine if you’ve been diagnosed with hypothyroidism.
After Delivery
Your thyroid hormone needs drop after the baby is born, so your levothyroxine dose will need to come back down. The general recommendation is to reduce to about two-thirds of the dose you were taking at the end of pregnancy. Women with subclinical hypothyroidism who had no goiter or TPO antibodies may be reduced to one-quarter of their final pregnancy dose, and some can stop the medication entirely.
Your provider will recheck your TSH about six to eight weeks after delivery to fine-tune the dose. If you were diagnosed with hypothyroidism for the first time during pregnancy, this postpartum period will help determine whether you need long-term treatment or whether the condition was specific to pregnancy. Women who are TPO-antibody positive are more likely to need ongoing medication.