Author: Dr Irfana Koita | Published: 14 October 2026
Yes, thyroid problems can cause infertility, but mostly when the thyroid is clearly underactive. An underactive thyroid (hypothyroidism) is linked to irregular periods, missed ovulation and lower markers of egg supply. A slightly raised TSH, or thyroid antibodies with normal thyroid function, shows a weaker and less consistent link.
This matters before IVF. A 2026 Lancet review found that large trials do not support giving thyroid tablets to women whose thyroid levels are normal, even when antibodies are present.
This guide uses ten studies from 2026 to explain thyroid and fertility in plain language. It covers what high TSH means for IVF, which TSH levels before IVF are considered fine, and what thyroid testing before pregnancy involves.
Unsure where to start? Book a free advisory call or explore the thyroid fertility test offered by IVF Matters.
Quick Glossary
| Term | What it means |
|---|---|
| Thyroid | A small butterfly shaped gland in your neck. It controls how your body uses energy. |
| TSH | Thyroid stimulating hormone. Your brain releases more of it when the thyroid is struggling. |
| Free T4 | The main thyroid hormone in your blood. Low levels mean an underactive thyroid. |
| Overt hypothyroidism | High TSH and low free T4. |
| Subclinical hypothyroidism | High TSH but normal free T4. A milder, earlier picture. |
| Thyroid antibodies | Immune proteins that mistakenly attack the thyroid. Found in autoimmune conditions such as Hashimoto's. |
| AMH | A hormone that estimates how many eggs you have left (your ovarian reserve). |
| Levothyroxine | A daily tablet that replaces thyroid hormone. |
Thyroid and Fertility: How They Are Connected
Your thyroid hormones help run your cycle and your eggs. Human eggs and the cells that feed growing egg follicles (granulosa cells) have receptors, or docking points, for thyroid hormone. In lab studies, thyroid hormone helps those cells grow (Iwase et al., 2026).
The brain adds a second link. When thyroid hormone is low, the brain sends out more of a signal called TRH. This raises TSH and also prolactin, a hormone that can stop ovulation. A 2026 review in the World Journal of Pharmaceutical Sciences describes this chain and advises that women planning pregnancy have thyroid hormones and prolactin checked.
The immune system is the third link. Hashimoto's thyroiditis, where the body attacks its own thyroid, affects women about seven times as often as men. Antibodies are found in 5 to 20% of women with normal thyroid function (Poppe et al., 2026).
Can Thyroid Problems Cause Infertility?
They can, but the strength of the link depends on the condition.
| Condition | What tests show | What research suggests |
|---|---|---|
| Overt hypothyroidism | High TSH, low free T4 | Clearly linked to cycle problems and lower egg supply. Treatment is standard. |
| Subclinical hypothyroidism | High TSH, normal free T4 | Mixed evidence. Many mild results return to normal on retesting. |
| Thyroid antibodies only | Positive antibodies, normal TSH | Linked to miscarriage and some subfertility. Treatment has not been shown to help. |
| Overactive thyroid | Low TSH, high hormones | Needs specialist control before trying to conceive. |
Hypothyroidism and Infertility: What Studies Show
Hypothyroidism infertility is more common than many people expect. Three small 2026 studies point the same way.
Prevalence and treatment. A study of 100 infertile women in India found thyroid problems in 46%: 33% subclinical and 13% overt. After starting levothyroxine, 84.6% of women with overt hypothyroidism conceived. The rates were 48.5% with subclinical and 44.5% in untreated women with normal thyroid function (Das et al., 2026). There was no control group, so this shows a link, not proof.
Egg supply. In 60 women with hypothyroidism, 53.3% had low AMH. Women with overt disease had lower AMH (1.5 against 2.2 ng/mL) and fewer follicles on ultrasound. Higher TSH went with lower AMH (a correlation of negative 0.41, which is a moderate link). Every woman had hypothyroidism, so there was no normal comparison group (Munmun et al., 2026).
Cycles and prolactin. A Uzbek study compared 88 women with hypothyroidism against 20 controls. AMH was lower (about 1.8 to 1.9 against 3.3 ng/mL) and prolactin much higher. Cycle problems affected 52% of women with autoimmune hypothyroidism, against 25% of controls (Nasirova et al., 2026). The groups were small and not perfectly matched.
Thyroid Antibodies and Fertility: Do They Matter?
Antibodies are more common in women with fertility problems, but treating them has not improved live birth rates.
A pooled analysis (a study combining many studies) in the Lancet review found antibodies in 20% of women with subfertility, against 13.3% of controls. They were also more common with endometriosis, PCOS and premature ovarian insufficiency.
The effect on egg supply looks small. Women with antibodies tend to have modestly lower AMH, not enough to suggest ovaries about to stop working. Genetic analyses suggest early ovarian failure may lead to thyroid autoimmunity, not the reverse (Iwase et al., 2026).
Three randomised trials (women assigned by chance to treatment or a dummy pill) tested levothyroxine in women with antibodies:
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POSTAL: 600 women having IVF. No effect on miscarriage or live birth.
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TABLET: 952 women. Live birth was 37.4% with levothyroxine and 37.9% with placebo.
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T4LIFE: 187 women with repeated miscarriage. Live birth was 50% against 48%.
High TSH and IVF: What Happens During Treatment
A higher TSH may lower IVF success, but the evidence is not yet firm.
A 2026 study followed 98 women with normal thyroid function having IVF. Women with TSH of 0.5 to 2.5 mIU/L had a 46.2% pregnancy rate. Women with TSH of 2.5 to 4.5 had 30.3%. That gap could be chance (p = 0.098), and antibodies made no clear difference (Dirgen Yakar et al., 2026). In the Uzbek study, TSH before IVF did not separate women who became pregnant from those who did not.
The Lancet review adds that a TSH above 4.0 mIU/L may affect fertilisation and embryo quality, and could improve with levothyroxine.
IVF itself can nudge TSH up. The drugs raise oestradiol, which temporarily lowers free T4. Women with antibodies or past thyroid surgery are most at risk. If you already take levothyroxine, your dose should be settled before stimulation, and thyroid function rechecked 2 weeks after a positive test. Our guide to IVF injections explains the drugs.
TSH Levels Before IVF: What Number Should You Aim For?
There is no single TSH number that suits everyone. The old target of below 2.5 mIU/L is now seen as a treatment goal, not a diagnosis.
| Guideline | Position |
|---|---|
| ASRM 2024 | Use the lab's own upper limit (about 4.12 mIU/L if an age based limit is unavailable). Advises against routine screening and routine levothyroxine to improve fertility. |
| ETA 2021 | Uses 4.0 mIU/L as a practical threshold in assisted reproduction. |
| ATA 2026 | Use local reference ranges, confirm with repeat tests, and individualise. |
Source: Alredha et al., 2026.
The same review warns against calling a TSH of 2.5 to 4.0 "subclinical hypothyroidism" in women who are not pregnant. The Lancet authors found no evidence for treating women whose TSH is in the normal range, and say aiming below 2.5 in all subfertile women is not recommended. It can mean overtreatment.
A mildly abnormal result should be repeated after 4 to 6 weeks. TSH often returns to normal alone.
Thyroid Testing Before Pregnancy: Who Needs It?
Thyroid testing before pregnancy is most useful if you have symptoms or a relevant history. Consider asking for it if you have:
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Irregular or absent periods
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Repeated miscarriage
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Failed IVF with no clear cause
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Unexplained infertility
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A family history of thyroid disease
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A known thyroid condition
A sensible order looks like this:
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TSH and free T4. These show whether the thyroid is working.
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Antibody test (TPO). Guidelines disagree. ESHRE (2022) advises it after repeated miscarriage. ASRM (2024) advises against routine antibody testing without symptoms.
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Repeat mild changes after 4 to 6 weeks.
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Read results alongside your AMH and cycle history.
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Agree a plan: monitor, treat, or see an endocrinologist (hormone specialist).
Thyroid Fertility Test at IVF Matters
The Thyroid Profile + Clinical Review costs £199 for the home kit or £238 for the London walk in option. It measures TSH, free T4, free T3, total T4, and TPO and thyroglobulin antibodies. You give a blood sample, results arrive through a secure portal, and a call with Dr Irfana Koita is included.
Pair it with an AMH test to see egg supply alongside thyroid function. Our guide to understanding fertility test results explains the numbers.
Thyroid Problems and Male Fertility
Thyroid imbalance may affect men too, though the evidence is earlier. A 2026 study compared 150 infertile men with 150 fertile men. The infertile group had higher TSH, lower thyroid hormones and higher body weight. Activity of the Y chromosome AZF genes, which sperm production needs, was roughly 40% lower (Bindhu et al., 2026). Gene activity was measured in blood, and the study shows association only. Read more in our guide to low sperm count treatment.
Conclusion: What the Research Tells Us
Across all ten studies, a clearly underactive thyroid affects fertility and should be treated. Milder changes matter less than most people fear, and tablets for normal results have not helped.
| Study | What it adds |
|---|---|
| Lancet review (Poppe et al.) | Antibodies are common in subfertility, but three large trials show levothyroxine does not raise live birth. |
| Wiley review (Iwase et al.) | Antibodies may lower AMH slightly. Thyroid hormone acts directly on eggs and follicles. |
| Springer review (Alredha et al.) | No single TSH cutoff exists. Guidelines differ, and TSH of 2.5 to 4.0 should not automatically count as disease. |
| EJMR study (Dirgen Yakar et al.) | In 98 women having IVF, a higher TSH showed a possible dip in pregnancy rates that could be chance. |
| India study (Das et al.) | 46% of infertile women had thyroid problems. Overt hypothyroidism had the highest conception rate after treatment. |
| Bangladesh study (Munmun et al.) | Higher TSH went with lower AMH, and overt disease looked worse than subclinical. |
| Uzbek study (Nasirova et al.) | Hypothyroidism went with lower AMH, more cycle problems and higher prolactin. |
| WJPS review (Bhargavi et al.) | Low thyroid hormone raises prolactin, which can stop ovulation. |
| De Gruyter study (Bindhu et al.) | In men, higher TSH went with lower activity of sperm genes. |
| Frontiers analysis (Qiao et al.) | Infertility is linked to more thyroid cancer diagnoses, but this does not prove a cause. |
Check your thyroid early, repeat mild results after 4 to 6 weeks, and treat overt hypothyroidism. If your TSH is normal, extra thyroid tablets are unlikely to help. Most of these studies are small, so decisions should be made with a specialist who knows your full history.
Frequently Asked Questions
Can Thyroid Problems Cause Miscarriage?
The Lancet review reports higher miscarriage and preterm birth rates in women with thyroid antibodies, even with normal thyroid function. Underactive thyroid adds risks such as high blood pressure in pregnancy. These are links, so discuss your own risk with a specialist.
Does Thyroid Autoimmunity Mean I Need ICSI Instead of IVF?
No. The Lancet review found no consistent difference in fertilisation between IVF and ICSI (injecting one sperm into an egg) in women with antibodies. No guidance recommends ICSI based on antibodies alone.
Can Selenium or Diet Lower Thyroid Antibodies?
The Lancet review found no high quality studies showing that diet, steroids or selenium improve pregnancy outcomes in thyroid autoimmunity. Speak to your doctor before starting supplements.
Should I Recheck My Thyroid If Antibodies Are Positive but TSH Is Normal?
Yes, regular monitoring is sensible. In the TABLET trial, at least 7% of such women developed hypothyroidism within 12 months.
Does Infertility Raise the Risk of Thyroid Cancer?
A 2026 analysis of nine studies and about 4.56 million people found a 37% higher thyroid cancer risk in people with infertility (RR 1.37). The studies varied a lot, and extra scans may explain some of the gap. It shows a link, not a cause (Qiao et al., 2026).
What Researchers Still Do Not Know
Most thyroid and fertility studies are small and hard to compare. No trial has tested levothyroxine only in women with secondary infertility (trouble conceiving after a previous pregnancy). Our guide to secondary infertility covers other causes. Treat a thyroid result as one piece of your fertility picture.
Take the Next Step
If you have thyroid concerns and want to conceive, a clear plan beats guesswork. Book a free advisory call or arrange a fertility consultation with Dr Koita.
Resources
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Poppe, Dhillon-Smith and Kahaly: Thyroid Autoimmunity and Assisted Pregnancy (Lancet Obstetrics, Gynaecology & Women's Health, 2026): Antibodies, IVF outcomes, trials and screening
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Iwase et al.: Autoimmune Thyroid Disease and Female Fertility (Reproductive Medicine and Biology, 2026): Ovarian reserve and mechanisms
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Alredha et al.: Subclinical Hypothyroidism in Secondary Female Infertility (Middle East Fertility Society Journal, 2026): TSH thresholds and guidelines
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Dirgen Yakar et al.: Thyroid Function and Antibodies in IVF Success (European Journal of Medical Research, 2026): 98 women having IVF
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Das et al.: Hypothyroidism in Infertile Women (Acta Medica International, 2026): Prevalence and treatment response
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Munmun et al.: Ovarian Reserve Among Infertile Women with Hypothyroidism (The Insight, 2026): TSH and AMH
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Nasirova et al.: Hypothyroidism and IVF by Autoimmune Status (Austrian Journal of Technical and Natural Sciences, 2026): Cycles, prolactin and AMH
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Bhargavi et al.: Thyroidism and Infertility (World Journal of Pharmaceutical Sciences, 2026): TRH and prolactin pathway
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Bindhu et al.: Thyroid, Obesity and Y Chromosome Genes in Male Infertility (Hormone Molecular Biology and Clinical Investigation, 2026): Case control study of 300 men
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Qiao et al.: Infertility and Thyroid Cancer Risk (Frontiers in Oncology, 2026): Meta analysis of nine studies
Explore More
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IVF Matters: Thyroid Profile + Clinical Review: Order the test and book the follow up call
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IVF Matters: AMH Test: Check your egg supply
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Understanding Your Fertility Test Results: How to read your numbers
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IVF Injections Explained: Types, schedule and side effects
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5 Low Sperm Count Treatment Options for Men: Next steps after a male factor result
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Secondary Infertility: Why You May Struggle to Conceive Again: Other hidden causes
