Thyroid and Fertility: What Patients Need to Know About the New 2026 Guidelines
Key Takeaways
- The American Thyroid Association released its first thyroid-in-pregnancy guideline update since 2017 in May 2026.
- TSH is the test that matters. Target before conception on levothyroxine: 0.5 to 2.5 mIU/L.
- A mild TSH elevation below 6 mIU/L gets retested before any treatment decision.
- Levothyroxine doesn’t help women with TPO antibodies and normal TSH: 3 trials showed no fertility or miscarriage benefit.
- Thyroid testing belongs in every infertility and recurrent pregnancy loss workup.
If you’re trying to conceive, undergoing fertility treatment, or have experienced recurrent pregnancy loss, you’ve probably heard your doctor mention thyroid testing. The thyroid is a small gland in your neck, and it has an outsized effect on ovulation, implantation, pregnancy, and your baby’s brain development.
In May 2026, the American Thyroid Association (ATA) published updated guidelines for thyroid disease in preconception, pregnancy, and postpartum care, the first full revision since 2017. Several recommendations changed, including who gets tested, when to treat mild abnormalities, and how to manage thyroid antibodies.
Markham Fertility Centre includes thyroid assessment in fertility and recurrent pregnancy loss workups, in line with these recommendations, and this guide explains what the changes mean for patients in plain language.
The Connection Between Thyroid and Fertility
The thyroid is a butterfly-shaped gland at the front of the neck. It produces 2 main hormones, T4 (thyroxine) and T3 (triiodothyronine), and both are regulated by TSH (thyroid-stimulating hormone), which is produced by the pituitary gland in the brain.
Thyroid hormones influence:
- Ovulation
- Development of the uterine lining
- Early embryo development
- Maintenance of pregnancy
- Fetal brain development
Fertility specialists watch the thyroid closely because during early pregnancy, the developing baby depends entirely on the mother’s thyroid hormone supply. The baby’s own thyroid gland does not start functioning until later, so adequate maternal thyroid hormone matters most during the first trimester.
Thyroid disorders are also common in women of reproductive age. Autoimmune thyroid disease, particularly Hashimoto’s thyroiditis, is among the most common autoimmune conditions affecting women in their 20s, 30s, and 40s. Some patients arrive at a fertility clinic already diagnosed. Others find out during fertility testing.
Who Should Have Thyroid Testing?
One of the bigger changes in the 2026 guidelines involves screening.
Older guidelines used a long list of risk factors to decide who gets tested. The update removed several of them, including maternal age, body mass index (BMI), and the number of previous pregnancies, because newer research showed they added little value in identifying thyroid disease.
The current risk factors that support testing include:
- A personal history of thyroid disease
- Positive thyroid peroxidase (TPO) antibodies
- Type 1 diabetes or other autoimmune disorders
- Previous thyroid surgery or radiation exposure
- A family history of autoimmune thyroid disease
- Previous pregnancy complications
- Infertility
- Recurrent pregnancy loss
If you are being evaluated for infertility or recurrent pregnancy loss, thyroid testing should be part of your workup.
Which Thyroid Test Matters Most?
TSH is the primary test across virtually all major fertility and pregnancy guidelines.
Doctors sometimes order a free T4 level as well, but free T4 becomes harder to interpret as pregnancy progresses, because pregnancy changes the proteins that transport thyroid hormone in the blood. When TSH and free T4 seem to tell different stories, guidelines generally recommend placing more weight on the TSH value.
Patients often ask whether thyroid levels fluctuate during a normal menstrual cycle. According to the updated guidelines, they don’t change significantly during a regular ovulatory cycle, so testing early or late in the month usually makes no meaningful difference to your results.
How Fertility Treatment Affects Your Thyroid
During IVF and other ovarian stimulation treatments, estrogen levels rise sharply. That rise:
- Increases thyroxine-binding globulin (TBG), the protein that carries thyroid hormones
- Speeds up the breakdown of thyroid hormones
- Places greater demand on the thyroid gland
As a result, TSH can climb during treatment.
This matters most for women who already have hypothyroidism and take levothyroxine. Their thyroid gland can’t simply produce more hormone on demand, so they may need a higher dose of medication during stimulation cycles.
For patients already on levothyroxine, the ATA recommends checking TSH once within the 6 to 12 weeks before starting ovarian stimulation, so thyroid function can be brought into range before treatment begins.
What TSH Level Is Ideal Before Pregnancy?
For women with hypothyroidism who take levothyroxine, the 2026 ATA guidelines recommend aiming for a TSH between 0.5 and 2.5 mIU/L before conception.
This target leaves room for the natural increase in thyroid hormone requirements once pregnancy begins. The same target applies specifically to women with infertility or recurrent pregnancy loss who are being treated with levothyroxine.
What Happens to Thyroid Medication During Pregnancy?
Most women with hypothyroidism need a higher levothyroxine dose during pregnancy.
The ATA recommends:
- TSH testing about every 6 weeks during the first half of pregnancy
- At least one test during the third trimester
- A repeat test 5 to 6 weeks after any dose change
Dose requirements typically evolve like this:
| Stage | Typical levothyroxine adjustment |
| By 12 weeks of pregnancy | Increase of about 25% |
| By 20 weeks of pregnancy | Increase of up to 50% |
| After delivery | Return to pre-pregnancy dose, with a check around 6 weeks postpartum |
Subclinical Hypothyroidism: A Major Change in Approach
Subclinical hypothyroidism means TSH levels are elevated while free T4 remains normal. Historically, many patients started levothyroxine after a single abnormal TSH result. The 2026 guidelines recommend slowing down.
Research has shown that many mild TSH elevations are temporary. In some studies, only about half remained abnormal when retested 1 to 3 weeks later.
The new approach splits on one threshold:
| TSH result | 2026 ATA recommendation |
| Below 6 mIU/L | Repeat the test to confirm the abnormality before making management decisions |
| 6 mIU/L or higher | Treat promptly. This level is considered overt hypothyroidism in pregnancy |
Why timing matters
The newest evidence suggests that if treating subclinical hypothyroidism helps, it helps early in pregnancy, particularly during the first trimester. Several large studies that started treatment later in pregnancy found no improvement in pregnancy outcomes or in child IQ at age 5.
Treatment that starts after the first trimester appears to miss most of its potential benefit, because that’s when the baby’s brain relies on maternal thyroid hormone.
What about IVF patients with mildly elevated TSH?
The ATA discusses a small study of women undergoing IVF or ICSI with subclinical hypothyroidism. Women who started levothyroxine at the beginning of treatment had higher implantation and live birth rates than women who did not. The study was small, and the guidelines rate the evidence as lower quality.
Because IVF cycles carry real emotional and financial weight, the guidelines note that women with persistently elevated TSH who are undergoing fertility treatment may benefit from discussing low-dose levothyroxine with their fertility specialist.
TPO Antibodies With Normal Thyroid Function: The Myth-Busting Update
The biggest single change in the 2026 guidelines concerns women who have positive TPO antibodies and a normal TSH. This is a common finding in fertility clinics, and for years some of these patients were offered levothyroxine in the hope it might improve fertility or reduce miscarriage risk.
The evidence is now clear. 3 large, high-quality randomized controlled trials found that levothyroxine did not improve fertility, reduce miscarriage rates, or prevent preterm birth in euthyroid women with thyroid antibodies. That result held for women trying to conceive naturally, for IVF patients, and for women with recurrent pregnancy loss.
The ATA therefore does not recommend routinely prescribing levothyroxine in this situation.
So what should TPO-positive patients do?
Monitoring still matters. Women who are TPO-positive have roughly a 7% to 9% chance of developing subclinical or overt hypothyroidism before or during pregnancy. The guidelines recommend periodic TSH monitoring every 6 months, while trying to conceive.
Antibody status has also lost its role as a tiebreaker. Previous guidelines leaned toward treating subclinical hypothyroidism when a patient was TPO-positive.
The 2026 guidelines state that TPO positivity should no longer be the deciding factor, because the difference in pregnancy risk between TPO-positive and TPO-negative patients with subclinical hypothyroidism appears relatively small. Treatment decisions now rest on the TSH level, the timing, and the patient’s preferences.
What about selenium and other supplements?
Patients frequently ask whether selenium can lower antibodies or improve outcomes. After reviewing the evidence, the ATA found no support for selenium, intravenous immunoglobulin (IVIG), glucocorticoids, or other immune-modulating therapies in euthyroid, antibody-positive women. These treatments are not recommended.
Taking Desiccated Thyroid or T3? Plan the Switch Before Pregnancy
The guidelines strongly recommend that women planning pregnancy switch to levothyroxine alone if they currently take desiccated thyroid extract, T3-only medication, or a T3/T4 combination.
The developing fetal brain relies on maternal T4 crossing the placenta and being converted into T3 within fetal tissues. T3 itself does not cross the placenta efficiently, so medications that provide proportionally more T3 and less T4 may leave the fetus short of what it needs.
If you take one of these medications and are planning pregnancy, discuss transitioning to levothyroxine with your physician well before attempting conception.
The Gray Zone: Why Guidelines Don’t Fully Agree on TSH 2.5 to 4
All major organizations agree that overt hypothyroidism should be treated. They differ on what to do when TSH sits between roughly 2.5 and 4 mIU/L, especially for patients with recurrent pregnancy loss. This is where Canadian patients will notice that different doctors reasonably take different approaches.
| Organization | Position on the gray zone |
| SOGC (Society of Obstetricians and Gynaecologists of Canada) | TSH target below 2.5 mIU/L for patients on levothyroxine trying to conceive. Increase levothyroxine after a positive pregnancy test, monitor every 4 to 6 weeks. Antibody status alone doesn’t change management when TSH is normal |
| CFAS (Canadian Fertility and Andrology Society) | No levothyroxine to prevent pregnancy loss when preconception TSH is below 4 mIU/L, regardless of antibody status. Antibody testing can still flag people at higher future risk who may benefit from closer monitoring |
| ASRM (American Society for Reproductive Medicine) | Screen with TSH, treat when TSH exceeds 4 mIU/L or the lab’s upper limit of normal. No routine antibody screening solely for recurrent pregnancy loss evaluation |
The evidence in this specific range is still limited and mixed, which is why reasonable specialists land in different places.
Iodine: The Nutrient That’s Easy to Overlook
The updated guidelines reinforce the role of iodine, the raw material for thyroid hormone.
| Stage | Recommended daily iodine |
| Planning pregnancy | About 150 mcg supplemental, ideally started at least 3 months before conception |
| Pregnancy and breastfeeding | About 250 mcg total, from food and supplements combined |
| Upper limit | No more than 500 mcg supplemental |
People at greater risk of low iodine intake include those who:
- Avoid dairy products
- Don’t use iodized salt
- Follow vegan diets
- Live in regions where food iodine content is lower
The ATA cautions against exceeding 500 micrograms of supplemental iodine daily, because excess iodine can itself cause thyroid dysfunction. Check your prenatal vitamin’s label for its iodine content.
Final Thoughts
Thyroid-related fertility issues are usually manageable. Experts still disagree on some treatment thresholds, but the fundamentals are shared: find thyroid disease early, monitor it properly, and avoid both under-treatment and over-treatment. A conversation with a fertility specialist can clarify exactly where you fit within the 2026 recommendations.
This article is for education only and does not replace medical advice from your own healthcare team. Thyroid management should always be individualized based on your medical history, lab results, and treatment plan.
Book a consultation at Markham Fertility Centre →
Frequently Asked Questions
Can I get pregnant if I have a thyroid problem?
Yes, in most cases. Both hypothyroidism and hyperthyroidism can interfere with ovulation and increase pregnancy risks, but both are treatable, and well-managed thyroid disease is compatible with healthy pregnancy. The key is identifying the problem and adjusting treatment before and during pregnancy.
What should my TSH be when trying to conceive?
If you have hypothyroidism and take levothyroxine, the 2026 ATA guidelines recommend a TSH between 0.5 and 2.5 mIU/L before conception. If you’re not on thyroid medication, a normal TSH for your lab’s reference range is the benchmark, and mild elevations should be confirmed with a repeat test before any treatment decision.
I have Hashimoto’s antibodies but my TSH is normal. Do I need medication?
Based on the 2026 guidelines, no. 3 large randomized trials showed levothyroxine does not improve fertility or reduce miscarriage in women with positive TPO antibodies and normal thyroid function. What you do need is periodic TSH monitoring, every 3 to 6 months while trying to conceive, because your risk of developing hypothyroidism later is around 7% to 9%.
Does a mildly high TSH mean I need treatment right away?
Usually not. Many mild TSH elevations normalize on their own, so the guidelines recommend repeating the test first when TSH is below 6 mIU/L. At 6 mIU/L or higher during pregnancy, treatment should start promptly.
Will IVF affect my thyroid levels?
It can. The estrogen surge during ovarian stimulation increases demand on the thyroid, and TSH may rise during treatment. Women already on levothyroxine should have their TSH checked in the 6 to 12 weeks before starting stimulation, and may need a dose increase during the cycle.
Sources
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Korevaar TIM, Leung AM, Alexander EK, et al. Thyroid. 2026;36(5):481-544.
- ATA Guidelines & Statements, American Thyroid Association.
- New 2026 ATA Thyroid Disease During Pregnancy Guideline Update, Guideline Central.
- American Thyroid Association 2026 Guidelines, PubMed.