Does Hashimoto's Cause Miscarriage? What the 2026 Guidelines Say

Hashimoto's thyroiditis is the most common autoimmune condition in women of reproductive age. It is also one of the least well-explained. Most women with a Hashimoto's diagnosis are told to keep their TSH in range and otherwise carry on. When they start trying to conceive, the conversation often stops there.

It should not stop there.

The relationship between Hashimoto's and pregnancy is more complicated than TSH management. Miscarriage risk is elevated even in women whose thyroid function looks completely normal on paper. The guidance on whether to treat has shifted dramatically over the past decade, and the 2026 American Thyroid Association guidelines made the clearest statement yet on who should and should not receive levothyroxine before and during pregnancy.

This article covers the evidence, the decision framework, and the specific bloodwork you need to track from pre-conception through the postpartum period.

How Hashimoto's affects fertility

Hashimoto's is not purely a thyroid problem. The antibodies your immune system produces against thyroid peroxidase (TPO antibodies) and thyroglobulin (TgAb) are markers of a broader immune imbalance that reaches beyond the thyroid gland and into the uterus itself.

A 2023 single-cell RNA transcriptome study found that autoimmune thyroid disease disrupts the immune environment of the endometrium during the implantation window (Xie et al., 2023). Specifically, uterine natural killer cells are reduced, cytotoxic T cell populations are elevated, and the Th1-to-Th2 ratio shifts in a way that reduces the uterus's tolerance for an implanting embryo. This is not a TSH problem. It is an immune problem that sits upstream of thyroid hormone levels entirely.

There is also a direct thyroid mechanism. In weeks 8 to 12 of pregnancy, human chorionic gonadotropin (hCG) surges and cross-stimulates the TSH receptor, essentially doubling the thyroid's workload. Women with Hashimoto's have a blunted response to this signal. Even if their TSH looks normal at baseline, their thyroid reserve is insufficient to meet the first-trimester demand (Korevaar, 2022).

Beyond miscarriage, Hashimoto's affects fertility in other ways:

• Irregular cycles and anovulation: Subclinical hypothyroidism elevates prolactin, which disrupts the LH/FSH axis and causes irregular or absent periods. In one study of 120 women with menstrual abnormalities, elevated TPO antibody levels were linked to amenorrhea in 50% of cases (Himabindu et al., 2024).
• Reduced ovarian reserve: Women with Hashimoto's show lower anti-Mullerian hormone (AMH) and higher FSH on average, suggesting diminished ovarian reserve independent of thyroid hormone status (Popa et al., 2025).
• Worse IVF outcomes: Clinical pregnancy rates in ICSI cycles drop from approximately 42% to 28% in women with thyroid autoimmunity compared to those without.

Does Hashimoto's cause miscarriage? The real numbers

Yes, but the mechanism is more nuanced than most people realise, and so is the treatment question.

The miscarriage rate gap is significant. A large meta-analysis found that TPO-positive women experience miscarriage at a rate of approximately 26.7% compared to 7.1% in TPO-negative women, roughly a 3.8-fold increase (cited in Korevaar, 2022). About 75% of those TPO-positive women are euthyroid, meaning their TSH is in the normal range. The elevated risk is real even when the thyroid function test comes back normal.

In women with recurrent pregnancy loss specifically, the data is similar. A 2022 prospective study found first-trimester miscarriage rates of 36.8% in TPO-positive women versus 24.0% in TPO-negative women (RR 1.54, 95% CI 1.05-2.24) among women with unexplained recurrent spontaneous abortion (Liu et al., 2022).

What the randomised trials actually show

Here is where the story gets more complicated, and where many online resources fall short of the current evidence.

Three large randomised controlled trials set out to test whether giving levothyroxine to euthyroid TPO-positive women would reduce miscarriage and improve live birth rates. None of them showed a significant benefit.

The TABLET trial enrolled 952 euthyroid TPO-positive women across 49 UK hospitals. Women received either 50 mcg levothyroxine daily or placebo from before conception through the end of pregnancy. Live birth rate: 37% in the levothyroxine group, 38% in the placebo group. No statistically significant difference (Dhillon-Smith et al., 2019).

The T4LIFE trial was a Phase 3 RCT specifically targeting women with two or more prior pregnancy losses who were TPO-positive and euthyroid. Live birth rate: 50% levothyroxine versus 48% placebo (RR 1.03; 95% CI 0.77-1.38). The trial was stopped early due to slow recruitment and still showed no benefit (van Dijk et al., 2022).

A 2025 meta-analysis pooled 8 RCTs and 1,645 women. The headline finding was a statistically significant 22% relative reduction in miscarriage (RR 0.78, p=0.035) in the levothyroxine group. However, live birth rate was not significantly improved overall (RR 1.05, p=0.097). There was a meaningful signal in the recurrent pregnancy loss subgroup specifically: live birth RR 1.21 (p=0.023), with no benefit in women without prior pregnancy loss (Provinciatto et al., 2025).

The 2026 ATA guidelines interpreted this body of evidence and issued a clear position: levothyroxine should not be offered to euthyroid TPO-positive women who are experiencing infertility, planning IVF, or who have had recurrent miscarriages (Korevaar et al., 2026). This was a formal stance based on three RCTs, not a conservative default.

The 2025 meta-analysis recurrent pregnancy loss signal is hypothesis-generating but does not change this recommendation. If you have had three or more losses, it is worth discussing with a reproductive endocrinologist who follows the current literature. But the population-wide guidance is clear.

Should you treat Hashimoto's before getting pregnant?

The answer depends on your TSH level, not your antibody status.

If your TSH is above 4.0 mIU/L: treatment with levothyroxine is recommended regardless of whether your TPO antibodies are positive or negative. Overt hypothyroidism and clinically elevated subclinical hypothyroidism carry clear risks in pregnancy, and the case for treatment is not contested.

If your TSH is between 2.5 and 4.0 mIU/L with positive TPO antibodies: this is where guidance has changed. The old 2017 ATA guidelines considered treatment "reasonable" in this zone. The T4LIFE RCT was designed precisely to test this, and found no benefit. The 2026 ATA guidelines have moved away from recommending treatment in this range. One 2024 propensity-matched study found that treating women with TSH between 2.5 and 10 mIU/L increased the risk of small-for-gestational-age infants in TPO-antibody-negative participants, and the risk of preterm birth in TPO-positive participants, a finding that makes over-treatment a real concern in both groups (Gao et al., 2024).

If your TSH is normal and your only finding is positive TPO antibodies: do not start levothyroxine. The three RCTs are consistent on this point, and the 2026 ATA guidelines are explicit. What you should do is monitor your TSH more frequently, because approximately 7 to 9% of euthyroid TPO-positive women will develop subclinical or overt hypothyroidism before or during pregnancy, at which point treatment becomes appropriate.

If you are planning IVF or ICSI: one Cochrane review found a potential benefit of levothyroxine for subclinical hypothyroidism in this context (RR 2.13 for live birth), though evidence quality was rated very low (Akhtar et al., 2019). For euthyroid TPO-positive women going through IVF, the evidence still does not support levothyroxine. Discuss the specifics with your fertility specialist.

If you are already on levothyroxine: your management pathway is different and is covered in the next section.

TSH targets in pregnancy: what the 2026 guidelines changed

TSH reference ranges shift during pregnancy, and the targets for treated Hashimoto's patients differ from the general reference intervals.

The first trimester sees a physiological drop in TSH because hCG cross-stimulates the TSH receptor. This means a TSH of 0.5 to 1.0 mIU/L in early pregnancy is often entirely normal, not a sign of overtreatment.

For women on levothyroxine, the clinical target is to keep TSH in the lower half of the pregnancy-specific reference interval throughout all three trimesters. The 2026 ATA guidelines use population-specific ranges rather than fixed cutoffs, with an upper limit of approximately 4.0 mIU/L in the first trimester for the general pregnant population (Korevaar et al., 2026).

The old guideline figures of "below 2.5 in the first trimester and below 3.0 in the second and third" were based on the 2011 ATA guidelines and have been progressively revised upward as larger population datasets emerged. The 2026 update reflects that non-pregnancy-specific reference intervals significantly overdiagnose subclinical hypothyroidism in pregnancy, labelling normal physiological TSH suppression as pathological (Warringa et al., 2026).

The practical implication for treated Hashimoto's patients: a TSH that was well-controlled at 1.8 mIU/L before pregnancy may drift toward 3.5 mIU/L in the second trimester even with a dose increase, and that may still be within the acceptable range. A TSH above 4.0 mIU/L in any trimester warrants a dose adjustment. A TSH that is persistently suppressed below 0.1 mIU/L suggests overtreatment, which also carries risks.

Always ask your laboratory or endocrinologist for trimester-specific reference ranges from the specific assay your lab uses, since there is meaningful variation between platforms.

What to do the day you get a positive pregnancy test

This section applies to women already taking levothyroxine for diagnosed hypothyroidism. If you are not currently on medication, the guidance is different (monitor TSH every four weeks, discussed below).

Levothyroxine requirements begin rising as early as week 5 of gestation (Alexander et al., 2004). By the time you have your first prenatal appointment, often around weeks 8 to 10, you have already had several weeks of rising demand. If your dose has not changed, you are likely behind.

The morning you get a positive test, before you call your doctor, add two extra levothyroxine tablets to your weekly schedule. This is not informal advice. It is the method endorsed by both the American Thyroid Association and the Endocrine Society, and it was the intervention arm of a dedicated RCT, the THERAPY trial, which found it prevented first-trimester TSH excursions better than a three-tablet increase without causing the over-suppression risk (Yassa et al., 2010).

The math: if you normally take seven tablets per week, taking nine gives you a 28.6% increase. This maps directly onto the ATA recommendation of 20 to 30%. Once you have made the change, contact your endocrinologist to schedule a TSH plus Free T4 check within four weeks.

The prenatal vitamin problem

Iron and calcium, both present in prenatal vitamins, substantially impair levothyroxine absorption. Ferrous sulfate raises TSH from the low normal range into the hypothyroid range in most women who take it alongside their levothyroxine (Campbell et al., 1992). Calcium carbonate reduces Free T4 and raises TSH in a similar way (Singh et al., 2000).

The fix is simple but requires consistency: take levothyroxine on an empty stomach as soon as you wake up. Wait at least four hours before taking your prenatal vitamin. Do not take them together even if it is more convenient. Many women discover their TSH control worsens significantly in the first trimester simply because they started taking a prenatal vitamin alongside their thyroid medication.

If you take desiccated thyroid (Armour Thyroid) or a compounded T3/T4

Switch to levothyroxine monotherapy before trying to conceive. This is a firm ATA recommendation (Alexander et al., 2017). The reason is not arbitrary preference for synthetic T4.

Free T3, the active component in desiccated thyroid and in compounded T3/T4 preparations like Cytomel, does not cross the placenta in meaningful amounts. The fetal brain is almost entirely dependent on maternal T4 crossing the placenta and being converted locally. Replacing T4 with a T3-heavy preparation reduces the maternal T4 available for placental transfer. This is not a paperwork concern; it is a fetal neurodevelopment concern.

How Hashimoto's affects fetal development

The fetal thyroid does not become functional until approximately weeks 12 to 14 of gestation. Before that point, every molecule of thyroid hormone driving fetal brain development comes from the mother.

Thyroid hormone receptors in the fetal brain are detectable from as early as week 8 and increase approximately eightfold between weeks 10 and 16. By the time these receptors are active, they need T4, and the only source is maternal.

Two landmark studies quantified what happens when maternal thyroid function is inadequate during this window.

Haddow et al. (1999) screened 25,216 pregnant women and identified 62 with elevated TSH. Their children were tested at ages 7 to 9. Children of untreated women averaged 7 IQ points lower than controls (p=0.005), and 19% scored below IQ 85 compared to 5% of controls. The effect was measurable across all 15 neuropsychological tests administered (Haddow et al., 1999).

Pop et al. (1999) studied 220 healthy pregnancies with thyroid function measured at 12 weeks. Infants born to mothers with Free T4 below the 10th percentile at 12 weeks scored 7.4 points lower on psychomotor development scales at 10 months of age. The association was specific to 12-week measurements. There was no significant effect for thyroid function measured at 32 weeks, confirming the first trimester as the critical window (Pop et al., 1999).

The conclusion from both studies: after gestational week 14, irreversible fetal brain changes from T4 insufficiency become increasingly likely. Getting TSH controlled before pregnancy and within the first four weeks of a confirmed pregnancy is not a precaution. It is the intervention that matters.

Do Hashimoto's antibodies cross the placenta?

Yes. Anti-TPO antibodies are IgG class and cross the placenta freely, with cord blood levels correlating almost perfectly with maternal levels (r=0.98) (Seror et al., 2014). This sounds alarming but is generally not a direct problem, because anti-TPO antibodies inhibit thyroid peroxidase activity but do not stimulate or block the TSH receptor. The fetal thyroid is not directly attacked by TPO antibodies.

The risk is indirect: if maternal Hashimoto's causes inadequate T4 production, it is the low maternal T4 that harms fetal development, not the antibodies themselves.

A small subset of Hashimoto's patients produce TSH receptor-blocking antibodies. These can inhibit fetal thyroid function directly, and babies born to mothers with high blocking antibody titers warrant neonatal thyroid monitoring. Standard neonatal heel-prick screening at day 3 to 4 catches most cases. For TPO-positive Hashimoto's patients without blocking antibodies, asking your neonatologist to check TSH between weeks 2 and 4 of life is reasonable, since transient neonatal TSH elevation can occur and typically resolves without treatment.

Bloodwork monitoring schedule through pregnancy

The core thyroid panel in pregnancy is TSH and Free T4 together. TSH alone is not sufficient because hCG-driven physiological changes in the first trimester can suppress TSH even when thyroid output is inadequate, creating a false sense of security. The Pop 1999 data specifically showed that Free T4 below the 10th percentile was a risk factor for adverse infant outcomes independent of TSH level.

Free T3 is not part of routine pregnancy thyroid monitoring (Stagnaro-Green et al., 2011). Rechecking TPO antibodies or TgAb during pregnancy is also not recommended once the diagnosis is established. Antibody titers fall during pregnancy due to immune tolerance, but the level does not guide dosing decisions.

Recommended schedule for women with Hashimoto's

If you are on levothyroxine:

• TSH + Free T4 every four weeks from pregnancy confirmation through week 20
• TSH + Free T4 at approximately 26 to 30 weeks
• TSH + Free T4 six weeks postpartum
• TSH at three months and six months postpartum (see postpartum section below)
• After stable, annual TSH monitoring long-term

If you are not on levothyroxine (euthyroid Hashimoto's):

• TSH + Free T4 every four weeks from pregnancy confirmation through week 20
• If TSH rises above the trimester-specific upper limit (approximately 4.0 mIU/L in the first trimester), initiate levothyroxine and transition to the schedule above
• TSH at six weeks postpartum, three months, and six months postpartum

If your hospital or lab provides trimester-specific TSH reference intervals calibrated to their assay, use those. National averages vary between assays and the reference ranges printed on standard lab reports are often based on the non-pregnant population.

Postpartum thyroiditis: what happens after birth

The immune suppression of pregnancy lifts sharply after delivery. For women with Hashimoto's, this immune rebound triggers a postpartum thyroid attack in 33 to 56% of cases, compared to 5 to 8% in the general population (Premawardhana et al., 2000); (Groer and Jevitt, 2014). If you had detectable TPO antibodies still present in your third trimester, your risk climbs toward 80%.

Postpartum thyroiditis (PPT) has three presentations:

• Thyrotoxic phase only (approximately 30%): elevated Free T4, suppressed TSH, onset 1 to 4 months postpartum
• Hypothyroid phase only (approximately 43%): elevated TSH, onset 4 to 8 months postpartum
• Biphasic (approximately 25%): thyrotoxic phase followed by hypothyroid phase

Most descriptions of PPT focus on the classic biphasic course, but most women who develop it will experience only one phase.

Symptoms and how to distinguish PPT from postpartum depression

The symptom overlap is substantial. Palpitations, anxiety, insomnia, and unexplained weight loss in months 2 to 3 postpartum can be the thyrotoxic phase. Fatigue, cold intolerance, mood changes, and cognitive fog in months 4 to 8 overlap almost exactly with the peak period for postpartum depression diagnosis.

Any postpartum woman presenting with these symptoms should have TSH and Free T4 checked before a postpartum depression diagnosis is finalised. They are not mutually exclusive. Undiagnosed hypothyroidism worsens depressive symptoms, and treating the thyroid disorder does not treat true PPD, but the two need to be separated through bloodwork.

The standard six-week postpartum check has only 53% sensitivity for PPT, because most cases present later. For TPO-positive women, the monitoring schedule above (six weeks, three months, six months) is the appropriate minimum.

Will it become permanent?

Approximately 20% of women who enter the hypothyroid phase of PPT will remain permanently hypothyroid per ATA estimates. In a 3 to 5 year follow-up study, 23% of women with PPT developed permanent hypothyroidism, rising to 46% at 6 to 7 year follow-up among those who were TPO-positive (Premawardhana et al., 2000).

PPT is best understood as the first clear clinical expression of underlying Hashimoto's disease rather than a separate condition. Most cases resolve within 12 months, but a PPT episode is a reliable signal that Hashimoto's will become clinically overt over the following 5 to 10 years. Annual TSH monitoring after a PPT episode is appropriate long-term.

Supplements: what has evidence

Selenium (200 mcg/day of selenomethionine)

Selenium is the most evidence-supported supplement for Hashimoto's, with the most relevant trial being pregnancy-specific.

Negro et al. (2007) randomised TPO-positive pregnant women to 200 mcg selenomethionine daily from the first trimester through the postpartum period versus placebo. Postpartum thyroid dysfunction occurred in 28.6% of the selenium group versus 48.6% of the placebo group. Permanent hypothyroidism: 11.7% versus 20.3% (Negro et al., 2007).

A 2024 meta-analysis of 29 cohorts and 2,358 participants confirmed that selenium reduces TPO antibody titers (SMD -0.96, CI -1.36 to -0.56) and TSH in untreated patients, with moderate evidence quality (Huwiler et al., 2024).

One important note: the 2026 ATA pregnancy guidelines stated there is "no data supporting the use of selenium to improve pregnancy outcomes" in thyroid disease. This appears to be a mismatch with the Negro 2007 trial, and likely reflects the ATA's threshold for making a broad population recommendation from a single RCT. The Negro trial data on postpartum thyroid dysfunction is real and replicated in other cohorts. Whether to take selenium in pregnancy should be a conversation with your endocrinologist, not a self-treatment decision.

Myo-inositol (600 mg/day)

The combination of myo-inositol 600 mg and selenomethionine 83 mcg daily reduced TPO antibodies by 44% and TgAb by 48% in a double-blind RCT of 48 women with Hashimoto's, compared to selenium alone (Nordio and Pajalich, 2013). A separate RCT found the combination normalised TSH in subclinical hypothyroidism patients where selenium alone did not (Nordio and Basciani, 2017).

Vitamin D

Observational data consistently shows an inverse correlation between vitamin D levels and TPO antibody titers, but pregnancy-specific RCTs in Hashimoto's patients are absent. The evidence supports correcting deficiency (below 20 ng/mL) rather than supplementing in the absence of confirmed deficiency.

What to avoid

High-dose iodine: the 2026 ATA recommends a total daily iodine intake of 250 mcg during pregnancy and breastfeeding. Supplements exceeding 500 mcg daily can trigger thyroid dysfunction, particularly in Hashimoto's patients where the thyroid is already inflamed. Kelp supplements, nascent iodine, and high-dose iodine protocols marketed for "thyroid support" are contraindicated.

Gluten-free diet (without celiac disease): a 2025 systematic review and meta-analysis found no significant effect on TSH, Free T3, or Free T4 in non-celiac Hashimoto's patients. One notable finding was a paradoxical increase in anti-TPO titers in the gluten-free groups, alongside a modest decrease in TgAb, suggesting the intervention does not reduce overall antibody burden (2025 meta-analysis). If you have confirmed celiac disease, a gluten-free diet is essential. Without that diagnosis, the evidence does not support it as a Hashimoto's intervention.

Practical recommendations

Before you start trying to conceive:

• Get TSH, Free T4, TPO antibodies, and TgAb tested if you have not recently
• If TSH is above 4.0 mIU/L, work with your endocrinologist to get it controlled before attempting conception
• If you are on desiccated thyroid, discuss switching to levothyroxine
• Tell your endocrinologist you are planning a pregnancy so they can set a monitoring schedule before you confirm
• If you are already on levothyroxine, confirm your current dose and establish when you will increase it

The day you confirm pregnancy:

• If on levothyroxine: take two extra tablets this week, and every week until your endocrinologist adjusts your dose based on TSH. Do not wait for an appointment.
• If not on levothyroxine: contact your endocrinologist to schedule a TSH and Free T4 draw within two weeks
• Switch your prenatal vitamin to a separate time from your levothyroxine. Four hours minimum between them.

Through each trimester:

• TSH and Free T4 every four weeks through week 20, then around week 28 to 30
• Use trimester-specific reference ranges, not the standard printed on your lab report
• Track results across appointments so you and your doctor can see the trend, not just the latest number

After delivery:

• Expect your levothyroxine dose to drop back toward your pre-pregnancy level within six to eight weeks postpartum
• Check TSH and Free T4 at six weeks, three months, and six months postpartum
• Be alert to symptoms of the thyrotoxic phase (palpitations, anxiety, unexpected weight loss at months 1 to 3) and the hypothyroid phase (fatigue, brain fog, mood changes at months 4 to 8)
• Any postpartum mood or energy symptoms should include a thyroid panel before a psychiatric diagnosis is made

Key takeaways

• Hashimoto's raises miscarriage risk even with normal TSH, because TPO antibodies are a marker of immune dysregulation that affects endometrial function directly.
• Three large randomised trials showed levothyroxine does not improve live birth rates or reduce miscarriage in euthyroid TPO-positive women. The 2026 ATA guidelines recommend against starting medication in this population.
• Treatment is appropriate when TSH exceeds 4.0 mIU/L, regardless of antibody status. The old 2.5 mIU/L treatment trigger for TPO-positive women is no longer supported.
• If you are already on levothyroxine, increase your dose by approximately 29% (two extra tablets per week) the day you confirm pregnancy, before waiting for a doctor's appointment.
• Iron and calcium in prenatal vitamins impair levothyroxine absorption. Take your thyroid medication four or more hours before your prenatal vitamin.
• Desiccated thyroid (Armour) is not appropriate in pregnancy. T3 does not cross the placenta in meaningful amounts. Switch to levothyroxine before conceiving.
• Monitor TSH and Free T4 every four weeks through the first half of pregnancy and around week 28 to 30.
• 33 to 56% of TPO-positive women develop postpartum thyroiditis. Check TSH at six weeks, three months, and six months postpartum. Do not rely on the six-week check alone.
• Selenium 200 mcg/day (selenomethionine) has the strongest evidence for reducing postpartum thyroid dysfunction in TPO-positive women, though the 2026 ATA guidelines do not yet make a population-wide recommendation.

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