Hashimoto’s thyroiditis does increase the risk of miscarriage, even when thyroid hormone levels appear normal. Women with the thyroid antibodies characteristic of Hashimoto’s face roughly two to four times the miscarriage risk compared to women without those antibodies. This elevated risk exists whether or not the condition has progressed to full hypothyroidism, which makes it a concern that many women and their doctors overlook.
Why Normal Thyroid Levels Don’t Eliminate the Risk
One of the most important things to understand about Hashimoto’s and pregnancy is that the problem isn’t just about thyroid hormone levels. Many women with Hashimoto’s are “euthyroid,” meaning their TSH and thyroid hormones fall within the normal range. Despite this, studies consistently show higher miscarriage rates in these women compared to healthy controls. The presence of thyroid peroxidase antibodies (TPO antibodies) appears to be a risk factor on its own, independent of whether the thyroid is producing enough hormone.
This matters because standard thyroid screening in pregnancy focuses on TSH. If your TSH comes back normal, the antibody question may never come up. But among women with unexplained recurrent pregnancy loss, the prevalence of thyroid autoimmunity ranges from 16% to 36%, far higher than in the general population. In one large prospective study of 576 women with unexplained recurrent pregnancy loss, 17.5% had thyroid autoimmunity despite normal thyroid function.
How Thyroid Antibodies Affect the Placenta
The connection between Hashimoto’s and miscarriage goes beyond hormones. TPO antibodies appear to directly interfere with how the placenta develops. Research using Doppler ultrasound and placental tissue analysis has found that women who test positive for TPO antibodies in the first trimester show higher rates of abnormal blood flow patterns in the uterine arteries and signs of poor placental development.
Specifically, the placenta depends on a process where maternal blood vessels in the uterine wall remodel to deliver blood efficiently. In women with thyroid autoimmunity, this remodeling is often incomplete. The result is a placenta with restricted blood supply, which can lead to miscarriage, fetal growth restriction, or preeclampsia later in pregnancy. Women with TPO antibodies and TSH levels at or above 2.5 had nearly six times the odds of severe blood flow problems in the placenta and over ten times the odds of fetal growth restriction compared to controls.
There’s also an immune component. Hashimoto’s signals an immune system that’s already somewhat overactive, and pregnancy requires the immune system to tolerate a genetically foreign embryo. Some researchers believe the same immune dysregulation that attacks the thyroid may also create a less hospitable environment for implantation and early pregnancy.
Antibody Levels Don’t Predict Individual Risk
You might assume that higher antibody levels mean a greater chance of miscarriage, but the data doesn’t support that. Studies comparing antibody titers in women who miscarried versus those who carried to term found no meaningful difference. In other words, it’s the presence of thyroid autoimmunity that matters, not the specific antibody number on your lab report. A woman with moderately elevated TPO antibodies faces a similar risk profile to one with very high levels.
Impact on Fertility Treatments
Thyroid autoimmunity also affects outcomes for women using assisted reproduction. In IVF cycles, women with thyroid autoimmunity had a clinical pregnancy rate of 34% compared to 38% without it. The gap was more pronounced in ICSI cycles: 28% versus 42%. Both procedures showed lower implantation rates and higher miscarriage rates in the thyroid autoimmunity group.
These differences persist even when thyroid hormones are well-controlled, reinforcing that the antibodies themselves, or the broader immune dysfunction they represent, play a role beyond what hormone replacement alone can fix.
Does Thyroid Medication Help?
Levothyroxine, the standard treatment for Hashimoto’s, clearly benefits women with overt hypothyroidism during pregnancy. The more complicated question is whether it helps women who have the antibodies but still have normal thyroid function.
A systematic review pooling data from nine studies found that levothyroxine supplementation in euthyroid, antibody-positive pregnant women did not significantly reduce miscarriage rates. The miscarriage risk ratio was 0.82, meaning a small trend toward benefit, but not statistically significant. When only randomized controlled trials were analyzed, the effect was even smaller. The review concluded there was no clear beneficial role for levothyroxine in affecting pregnancy outcomes for TPO-positive women with normal thyroid function.
That said, thyroid demands increase substantially during pregnancy, and women with Hashimoto’s have less thyroid reserve to meet that demand. TSH targets during pregnancy are generally tighter than outside of pregnancy. The American Thyroid Association recommends using population-specific reference ranges when available, with a general upper TSH limit of 4.0 during pregnancy when local data doesn’t exist. Many clinicians still aim for TSH below 2.5 in the first trimester for women with known thyroid autoimmunity, though the evidence base for that specific cutoff is debated.
Selenium and Other Considerations
Selenium has drawn interest because it plays a role in thyroid function and immune regulation. Supplementation for six months or longer has been shown to reduce TPO antibody levels in people with autoimmune thyroiditis. In pregnancy specifically, a couple of small trials found that selenium supplementation lowered antibody levels and reduced the risk of postpartum thyroiditis.
However, the evidence that selenium prevents miscarriage is thin. Clinical trials in pregnant women with thyroiditis have not found significant differences in adverse pregnancy outcomes between selenium and placebo groups. Low selenium status on its own has been linked to preterm delivery and low birth weight, so adequate selenium intake matters during pregnancy regardless of thyroid status, but it’s not a proven intervention for reducing Hashimoto’s-related miscarriage risk.
One trial using a nutritional dose of 60 micrograms daily found no benefit over placebo for reducing antibody levels. Higher doses of 83 to 200 micrograms daily showed more consistent antibody reductions, but the clinical impact on pregnancy outcomes remains unclear. Side effects were minimal, limited mostly to mild stomach discomfort.
What This Means Practically
If you have Hashimoto’s and are pregnant or planning to become pregnant, the most important step is making sure your thyroid function is monitored closely throughout pregnancy. TSH should be checked early in the first trimester and tracked regularly, since your thyroid will need to work harder as the pregnancy progresses. Women with Hashimoto’s who are already on levothyroxine often need a dose increase of 25% to 50% during pregnancy.
If you’ve experienced recurrent miscarriages without explanation, thyroid antibody testing is worth pursuing even if your TSH has always been normal. The connection between thyroid autoimmunity and pregnancy loss is strong enough that it should be part of any workup for unexplained recurrent loss. Knowing your antibody status won’t change the biology, but it gives your care team a clearer picture and allows for closer monitoring during the highest-risk early weeks of pregnancy.