Can Hypothyroidism Cause Miscarriage?

Hypothyroidism increases the risk of miscarriage, and the connection holds for both full-blown and milder forms of the condition. A large prospective study found that women with subclinical hypothyroidism had roughly three times the odds of miscarriage compared to women with normal thyroid function, and those odds climbed further when thyroid antibodies were also present. The relationship is not as simple as “low thyroid equals pregnancy loss,” though, because the type of thyroid problem, the presence of autoimmune markers, and the timing of treatment all shape the actual risk a given pregnancy faces.

Why Pregnancy Puts Extra Pressure on Your Thyroid

Your thyroid gland works harder during pregnancy than at almost any other time. The placenta produces human chorionic gonadotropin (hCG), a hormone that directly stimulates the thyroid to ramp up production of thyroid hormones. At the same time, rising estrogen increases levels of a binding protein that soaks up circulating thyroid hormone, so the gland has to compensate by making even more. On top of that, the developing baby depends entirely on the mother’s thyroid hormones during the first trimester, before its own thyroid starts functioning around week 12.

1PubMed Central. Thyroid hormone dysfunction during pregnancy: A review

A healthy thyroid can keep up with these demands. But if you already have a sluggish thyroid or borderline function, pregnancy can push you over the edge into genuine hypothyroidism. This is one reason why thyroid problems that never caused symptoms before pregnancy sometimes become clinically relevant once conception occurs. The hCG-thyroid interaction is well established: hCG has significant thyroid-stimulating activity once it binds to thyroid cell membranes, which is why thyroid hormone levels naturally fluctuate through the trimesters in proportion to hCG levels.

2PubMed. The role of hCG in regulation of the thyroid gland in normal and abnormal pregnancy

What the Numbers Look Like for Subclinical Hypothyroidism

Subclinical hypothyroidism means your TSH (the signal your brain sends to push the thyroid to work harder) is elevated, but your actual thyroid hormone levels still fall within the normal range. You feel fine, you have no symptoms, and outside of pregnancy it often gets a shrug from doctors. During pregnancy it is a different story. A prospective cohort study that followed women from early pregnancy onward found that those with more pronounced subclinical hypothyroidism had a miscarriage rate of about 7%, compared to roughly 2% in women with normal thyroid function. When subclinical hypothyroidism appeared alongside thyroid antibodies, the rate climbed to 10-15%.

3PubMed Central. Maternal Subclinical Hypothyroidism, Thyroid Autoimmunity, and the Risk of Miscarriage: A Prospective Cohort Study

A review in the Indian Journal of Endocrinology and Metabolism similarly concluded that pregnant women with subclinical hypothyroidism or thyroid antibodies face an increased risk of complications, with miscarriage prominent among them, alongside preeclampsia and perinatal mortality.

4PubMed Central. Recurrent pregnancy loss in patients with thyroid dysfunction

The risk is real, but context matters. A 7% miscarriage rate in the subclinical hypothyroidism group still means the large majority of those pregnancies did not end in miscarriage. Subclinical hypothyroidism raises the odds meaningfully, but it is far from a guarantee of a bad outcome.

Thyroid Antibodies as a Separate Risk Factor

One of the more complicated aspects of this topic is that thyroid autoimmunity, meaning the presence of antibodies that attack the thyroid (most commonly anti-TPO antibodies), appears to carry its own independent miscarriage risk. This holds even in women whose thyroid hormone levels look perfectly normal. The association between thyroid antibody positivity and spontaneous miscarriage was first reported in 1990, and a substantial body of literature has confirmed it since.

5PubMed Central. Thyroid antibodies and miscarriage: where are we at a generation later?

A prospective cohort study of euthyroid women (meaning their TSH and thyroid hormones were within normal range) found that elevated TPO antibody levels were associated with roughly two and a half times the odds of miscarriage compared to antibody-negative women.

6PubMed Central. Impact of Thyroid Autoantibodies on Pregnancy Outcomes in Euthyroid Women: A Prospective Cohort Study

Why antibodies alone would raise miscarriage risk even when thyroid function tests look normal is still debated. One theory is that thyroid autoimmunity signals a more generalized immune dysregulation that can interfere with the delicate immune tolerance the body must maintain to sustain a pregnancy. Another possibility is that antibody-positive women have subtly reduced thyroid reserve: their gland can keep up under normal conditions, but when pregnancy ramps up the demand, it falls short in ways that conventional lab cutoffs do not capture. This distinction between “hypothyroidism” and “thyroid autoimmunity” matters practically, because it means a normal TSH result does not necessarily give the all-clear if antibodies are present.

How Thyroid Hormones Support Early Pregnancy

The mechanisms connecting thyroid function to early pregnancy survival run through several pathways. Thyroid hormones play a direct role in the placenta’s development. They influence the proliferation and differentiation of trophoblast cells, which are the cells that form the outer layer of the embryo and eventually build the placenta. Thyroid hormones also support trophoblast invasion of the uterine wall, the process by which the embryo anchors itself and establishes a blood supply, and they promote the growth of new blood vessels in the uterine lining.

7PubMed. The interplay between thyroid hormones and the placenta: a comprehensive review

There is also an indirect route through progesterone. Thyroid hormone stimulates progesterone release from ovarian cells during the luteal phase of the menstrual cycle, and progesterone is critical for maintaining the uterine lining in the earliest weeks of pregnancy before the placenta takes over hormone production. Research has shown that thyroid hormone deficiency is associated with reduced progesterone secretion, and that adding the thyroid hormone T3 to ovarian cells in the lab significantly boosts their progesterone output.

8PubMed. Thyroid hormone stimulates progesterone release from human luteal cells by generating a proteinaceous factor

So when thyroid hormones are insufficient, the early pregnancy can be undermined in multiple ways: poor placental development, weak implantation, and inadequate progesterone support. Each of these could independently raise the chance of miscarriage, and in practice they likely compound one another.

Does Levothyroxine Treatment Reduce the Risk?

If hypothyroidism raises miscarriage risk, the logical question is whether treating it with thyroid hormone replacement (levothyroxine) brings that risk back down. The answer depends heavily on who is being treated and why.

For women with subclinical hypothyroidism, the evidence is encouraging. A systematic review and meta-analysis found that pregnant women with subclinical hypothyroidism who received levothyroxine had about 45% lower odds of pregnancy loss compared to untreated controls.

9PubMed Central. Pregnancy and Neonatal Outcomes With Levothyroxine Treatment in Women With Subclinical Hypothyroidism Based on New Diagnostic Criteria: A Systematic Review and Meta-Analysis

For women with recurrent pregnancy loss specifically, the picture is similar when subclinical hypothyroidism is present. A meta-analysis focused on this population found that levothyroxine increased the live birth rate and reduced the miscarriage rate in women with both subclinical hypothyroidism and TPO antibody positivity.

10PubMed. Effect of levothyroxine on the pregnancy outcomes in recurrent pregnancy loss women with subclinical hypothyroidism and thyroperoxidase antibody positivity: a systematic review and meta-analysis

Here is where it gets complicated: for women who have thyroid antibodies but normal thyroid function, treatment with levothyroxine has not shown a clear benefit. A well-designed trial published in the New England Journal of Medicine gave levothyroxine to women with TPO antibodies before conception and found no difference in live birth rates or miscarriage rates compared to placebo.

11PubMed. Levothyroxine in Women with Thyroid Peroxidase Antibodies before Conception

The T4LIFE trial, another rigorous placebo-controlled study, reached the same conclusion for euthyroid women with recurrent pregnancy loss who were TPO antibody positive. Live birth rates were virtually identical between the levothyroxine and placebo groups (50% vs 48%), and the researchers explicitly advised against routine levothyroxine use in this population.

12PubMed. Levothyroxine in euthyroid thyroid peroxidase antibody positive women with recurrent pregnancy loss (T4LIFE trial)

The takeaway is an important distinction: levothyroxine helps when thyroid function itself is impaired, but it does not appear to fix the problem when antibodies are present and the thyroid is working adequately. Whatever excess risk thyroid antibodies carry in euthyroid women, it is not simply a matter of insufficient thyroid hormone, so adding more thyroid hormone does not solve it.

13PubMed Central. Does levothyroxine improve pregnancy outcomes in euthyroid women with thyroid autoimmunity undergoing assisted reproductive technology?

TSH Cutoffs in Pregnancy Are Not Straightforward

Part of the difficulty in managing thyroid function during pregnancy is deciding what counts as “abnormal.” The TSH ranges your doctor uses outside of pregnancy do not cleanly apply during pregnancy. Early guidelines from the American Thyroid Association recommended tight cutoffs: a first-trimester upper limit of 2.5 mIU/L. This was far lower than the standard non-pregnant upper limit of around 4.0–4.5 mIU/L, and it meant a lot of women were suddenly classified as having subclinical hypothyroidism.

Subsequent research and meta-analyses found that these strict cutoffs may have been overcautious, and in 2017 the ATA revised its guidance, recommending an upper cutoff of 4.0 mIU/L when population-specific reference ranges are not available.

14PubMed Central. Thyroid-Stimulating Hormone Values in Pregnancy: Cutoff Controversy Continues?

Even that number comes with an asterisk. TSH reference ranges vary substantially between populations and between the assays used to measure them, which is why the main recommendation from both the ATA and other bodies is for each region to develop its own pregnancy-specific reference ranges based on local data.

15Clinical Chemistry. A-338 TSH Reference Range in the First Trimester in a Iodine Sufficient Population of Pregnant Women Attended by the Public Health System in Curitiba South of Brazil

For you as a patient, this means two things. First, a TSH of 3.5 in the first trimester might be considered fine by one set of guidelines and borderline high by another. Second, context matters more than any single number: a TSH of 3.5 with positive TPO antibodies is treated differently than a TSH of 3.5 without them. If you are concerned about your thyroid during pregnancy, asking your doctor which reference range they are using and whether your antibody status has been checked gives you a clearer picture than looking at the number alone.

Recurrent Pregnancy Loss and Thyroid Dysfunction

For women who have experienced two or more miscarriages, thyroid problems are a recognized contributing factor. A systematic review and meta-analysis found that the prevalence of subclinical hypothyroidism among women with recurrent pregnancy loss was about 13%, and thyroid autoimmunity was roughly twice as common in women with recurrent loss compared to controls.

16Fertility and Sterility. Subclinical Hypothyroidism and Thyroid Autoimmunity in Recurrent Pregnancy Loss: A Systematic Review and Meta-Analysis

A case-control study from a tertiary care hospital compared women with recurrent pregnancy loss to matched controls and found thyroid dysfunction in about 36% of the loss group versus 19% of controls. Subclinical hypothyroidism was the most common abnormality, present in about a quarter of women with recurrent loss.

17International Journal of Reproduction, Contraception, Obstetrics and Gynecology. Recurrent pregnancy loss and thyroid abnormalities: a case control study from a tertiary care hospital in Delhi

These numbers do not mean thyroid problems cause a quarter of all recurrent miscarriages. Recurrent pregnancy loss has many possible contributors, including chromosomal abnormalities, uterine structural problems, blood clotting disorders, and immune factors. But thyroid dysfunction is one of the more treatable causes on that list, which is why most clinical guidelines recommend checking thyroid function as part of the evaluation after recurrent losses.

The Screening Debate

Whether every pregnant woman should have her thyroid tested, or only those with risk factors, remains one of the more contentious questions in obstetric endocrinology. The case for universal screening is straightforward: targeted screening based on risk factors (family history of thyroid disease, history of autoimmune conditions, prior pregnancy complications) misses a large number of affected women. A meta-analysis found that case-based screening may miss up to 49% of pregnant women with thyroid dysfunction.

18PubMed. Universal screening versus selective case-based screening for thyroid disorders in pregnancy

An evidence-based review concluded that universal screening appears to be the most reasonable approach, noting that targeted case finding missed 42% of women who would have needed levothyroxine.

19PubMed Central. Controversial Screening for Thyroid Dysfunction in Preconception and Pregnancy: An Evidence-Based Review

Despite this evidence, many guidelines still stop short of recommending universal screening. A 2025 review in Clinical Medicine noted that while universal screening is not formally recommended, targeted case finding in women at risk is advised.

20PubMed Central. Controversies in thyroid disease management in pregnancy

The hesitation comes partly from the fact that, as the levothyroxine trials showed, identifying and treating thyroid antibodies in euthyroid women does not clearly improve outcomes. If screening finds a problem you cannot effectively treat, the cost-benefit calculation shifts. Still, for subclinical and overt hypothyroidism, treatment does appear to work, and a simple blood test can catch it. If you are planning a pregnancy and have not had your thyroid checked, it is a reasonable test to request, particularly if you have risk factors like a family history of thyroid or autoimmune disease.

Beyond Miscarriage: Preeclampsia and Child Development

The consequences of untreated hypothyroidism in pregnancy extend beyond miscarriage. A large Danish study found that women with hypothyroidism had higher rates of preeclampsia: about 3% compared to 2.2% in women without thyroid problems. The risk was most pronounced in women newly diagnosed with hypothyroidism during pregnancy, where preeclampsia rates reached 4.2%. Among women with very high TSH levels (6 mIU/L or above), the preeclampsia rate was roughly 6%, more than double the background rate.

21PubMed Central. Maternal hypothyroidism and the risk of preeclampsia: a Danish national and regional study

There are also potential effects on the child’s development. A systematic review and meta-analysis found that maternal subclinical hypothyroidism was associated with roughly twice the odds of indicators of intellectual disability in offspring. Maternal hypothyroxinemia (low free T4 with normal TSH) carried a somewhat smaller but still statistically significant increase in risk.

22PubMed Central. Maternal thyroid hormone insufficiency during pregnancy and risk of neurodevelopmental disorders in offspring: A systematic review and meta‐analysis

A prospective cohort study that tracked children’s development found that neurodevelopmental scores were actually lower in children born to mothers with subclinical hypothyroidism than in those born to mothers with overt hypothyroidism, which the authors attributed to the fact that overt hypothyroidism is more likely to be detected and treated.

23PubMed. Impact of levothyroxine therapy for maternal subclinical and overt hypothyroidism on early child neurodevelopment: A prospective cohort study

That finding is a good illustration of a recurring theme: it is the undiagnosed, untreated thyroid problems that tend to cause the most trouble. Overt hypothyroidism, severe enough to produce obvious symptoms and lab abnormalities, gets caught and treated. Subclinical hypothyroidism often does not, which is why the screening question matters so much.

Iodine, Environmental Chemicals, and the Father’s Thyroid

Iodine is the essential raw material for making thyroid hormones, so iodine deficiency has long been considered a risk factor for thyroid dysfunction in pregnancy. A systematic review noted that pregnant women are susceptible to changes in iodine status, which can contribute to hypothyroidism among other problems.

24PubMed Central. Implications of iodine deficiency by gestational trimester: a systematic review

However, the direct link between iodine deficiency and miscarriage is less clear than you might expect. A prospective cohort study that measured urinary iodine levels and tracked pregnancy outcomes found no significant difference in pregnancy loss rates between women with adequate iodine and those with mild, moderate, or even severe deficiency.

25PubMed Central. Pregnancy Loss and Iodine Status: The LIFE Prospective Cohort Study

This does not mean iodine does not matter. Severe iodine deficiency is rare in countries with iodized salt programs, so most of the study participants may have had sufficient baseline levels even in the “deficient” group. The evidence is stronger for iodine’s effect on fetal brain development than for a direct miscarriage link. Making sure you get adequate iodine through diet or prenatal vitamins is still standard advice during pregnancy.

Environmental endocrine disruptors are another area of growing interest. Exposure to certain chemicals during pregnancy has been linked to complications including spontaneous abortion, and some of these chemicals are known to interfere with thyroid function.

26PubMed Central. Endocrine disruptors and pregnancy

One question that rarely comes up is whether the father’s thyroid status matters. A study examining paternal subclinical hypothyroidism in couples undergoing IVF found that it did not significantly affect fertilization rates, embryo quality, or early miscarriage rates.

27PubMed. Paternal Subclinical Hypothyroidism Affects the Clinical Outcomes of In Vitro Fertilization/Intracytoplasmic Sperm Injection

So while paternal thyroid health can affect sperm quality and fertility in other ways, the miscarriage question appears to be primarily about the maternal thyroid environment the pregnancy develops in. The fetus depends on the mother’s thyroid hormones for its early development, and it is the mother’s thyroid capacity that pregnancy pushes to its limits.