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Unexplained Infertility: Is your Thyroid the missing piece?
- Nicole Peasnell
- Jul 30
- 6 min read
For many women, an undiagnosed or under-treated thyroid issue is a significant piece of the unexplained infertility puzzle. That's because your thyroid hormones help regulate ovulation, cycle regularity and implantation, so even a mild imbalance, whether it's an underactive thyroid, an overactive thyroid or an autoimmune thyroid condition, could quietly get in the way of falling pregnant, staying pregnant, or having an embryo transfer succeed.
The tricky part is that standard testing checks basic blood markers relating to thyroid health, but there are a number of other markers which help to reveal much more about your thyroid and immune health. This can mean people are told "everything's normal", when only part of the thyroid picture has been explored.
If you've been trying to conceive for a while, have had one or more unsuccessful embryo transfers, or have been given the label "unexplained infertility", it may be worth getting a thorough look at your thyroid health.
Why your thyroid matters when you're trying to conceive
Thyroid hormones are among the most important regulators of female reproductive function, directly influencing ovulation, cycle regularity and your ability to maintain a pregnancy, a connection confirmed by research published in Frontiers in Endocrinology¹.
Your thyroid is a small gland with a big job, also producing hormones that affect your metabolism and energy.
If your thyroid isn't working the way it should, your body may struggle to
ovulate consistently,
to build a healthy uterine lining for implantation, or
to sustain a pregnancy once it begins.
This is why thyroid function is relevant whether you're trying to conceive naturally, going through IUI or IVF, or trying to understand why a transfer wasn't successful or a pregnancy didn't continue.
When your Thyroid is underactive
An underactive thyroid, known as Hypothyroidism, means your body isn't producing or using enough thyroid hormone to keep things running smoothly. You might recognise the signs already:
Exhaustion that sleep doesn't fix.
Cycles that are irregular or heavier.
Increased hair loss or thinning.
Dry skin.
Weight that won't shift no matter what you do.
Brain fog that makes work and daily life feel harder than it should.
What often gets missed is that Hypothyroidism isn't only about how much hormone your thyroid produces. It's also about whether your cells can actually take up and convert that hormone, which can be compromised even when production looks adequate on paper. This is one of the reasons some women feel unmistakably Hypothyroid despite lab results looking clear.
It's exhausting trying to plan your life, let alone a pregnancy, around symptoms that never quite add up. A 'normal' TSH result can be reassuring on paper, but it doesn't always tell the full story of what's going on for you.
When your Thyroid is overactive
An overactive thyroid, or Hyperthyroidism, is less commonly discussed in the fertility space but is just as important. Here your thyroid is producing too much hormone, which can show up as a racing heart, anxiety, unexplained weight loss, heat intolerance and shorter or irregular cycles.
Left unaddressed, it may be a contributing factor to fertility challenges for some women, so it's worth just as much attention as an underactive thyroid.
Autoimmune Thyroid conditions and Fertility
Thyroid autoimmunity, where the immune system mistakenly damages the thyroid gland, is a common but often overlooked cause of poor thyroid function. Hashimoto's thyroiditis (linked to underactive thyroid) and Graves' disease (linked to overactive thyroid) both fall into this category.
This is far more common than most women I see in clinic realise. A 2023 review in Frontiers in Endocrinology2 reports that thyroid autoimmunity affects close to 1 in 10 women of reproductive age and is seen more often in women dealing with unexplained infertility or polycystic ovary syndrome (PCOS/PMOS) than in the general population.
Thyroid antibodies, specifically Thyroid peroxidase (TPO) and Thyroglobulin antibodies (TgAb), can be elevated years before TSH ever moves outside the normal range. The same review confirms a well-documented link between these antibodies and a higher risk of pregnancy loss, along with a greater likelihood of fertility challenges, even in women whose thyroid hormone levels test completely normal.
In some cases this exposes the missing piece for women who have been told their thyroid is fine but are still facing unexplained infertility, repeated unsuccessful transfers, or recurrent miscarriage.
Why "normal" isn't always the full story
Two women can both have a technically "normal" TSH result and be in completely different positions when it comes to their thyroid health and their reproductive readiness. Reference ranges tell you whether a result sits within a broad population range, not whether it's optimal for you, or for conception.
This lines up with what the research is finding too. The Tańska et al. review2 notes that simply topping up thyroid hormone with medication hasn't reliably improved pregnancy outcomes in women with thyroid autoimmunity, even in large clinical trials. This suggests thyroid autoimmunity can reflect a broader pattern involving the immune system, not just a straightforward hormone shortfall, which is exactly why looking at hormone levels in isolation can leave so much of the picture unexplained.
"In my experience working with many women in the fertility and thyroid space, I believe we need to go beyond standard thyroid testing, to understand not just production but conversion, utilisation and immune involvement, because that's what truly impacts your metabolism, energy and fertility."
- Nicole Peasnell, Fertility Naturopath, Brisbane
Thyroid antibodies and recurrent Miscarriage
If you've experienced miscarriage or difficulty conceiving, this connection is worth knowing about. The link between thyroid antibodies and pregnancy loss, including recurrent pregnancy loss, is one of the more consistently documented findings in this area of research2, even in women whose thyroid hormone results otherwise look completely normal.
It's a strong reason to ask for thyroid antibody testing specifically, rather than assuming a normal TSH result rules thyroid health out as a factor.
Unexplained Infertility and Thyroid health
Optimal thyroid function can help to support regular ovulation, a receptive uterine lining for implantation, and the ongoing hormonal environment a pregnancy needs to progress safely.
A 2025 review published in Nutrients3 concluded that preconception programmes focused on positive lifestyle and nutritional choices should be considered first-line treatment for unexplained infertility, and thyroid health sits squarely within that picture.
This is exactly why looking at the full thyroid picture, not just TSH, matters so much whether you're planning your first cycle, preparing for another transfer, or trying to understand what happened last time.
If you've been told everything looks normal
Standard thyroid testing gives a helpful first snapshot, but there's often more going on beneath a clear result, which is why so many women are told everything's normal despite still facing infertility, an unsuccessful transfer, or miscarriage. A deeper thyroid assessment looks at production, conversion, cellular utilisation and immune involvement, not just TSH on its own.
Ready for more answers?
With over 20 years experience, I'm a Brisbane based Naturopath for Fertility, Thyroid and Hormone health, working with women and couples in-clinic and via Telehealth across Australia and New Zealand who want to look beyond 'everything's normal'.
If any of this sounds familiar, come and read more about my approach to Fertility, including the Thyroid markers I look for and my programs, Thyroid DECODED for thyroid support, and ALIGN Fertility Program, my Fertility and preconception care program for those trying to conceive.
REFERENCES
1. Bucci I, Giuliani C, Di Dalmazi G, Formoso G, Napolitano G. Thyroid Autoimmunity in Female Infertility and Assisted Reproductive Technology Outcome. Frontiers in Endocrinology. 2022;13:768363. doi.org/10.3389/fendo.2022.768363
2. Tańska K, Gietka-Czernel M, Glinicki P, Kozakowski J. Thyroid Autoimmunity and Its Negative Impact on Female Fertility and Maternal Pregnancy Outcomes. Frontiers in Endocrinology. 2023;13:1049665. doi.org/10.3389/fendo.2022.1049665
3. Donato M, Capalbo A, Morizio E, Fratini RM, Pilenzi L, D'Antonio F, Stuppia L, Vitacolonna E, Gatta V, Konstantinidou F. The Role of Lifestyle Intervention in Female Fertility, a Modifiable Factor for Preconception Health. Nutrients. 2025;17(13):2101. doi.org/10.3390/nu17132101
MEDICAL DISCLAIMER
The information in this article is general in nature and provided for educational purposes only. It is not intended as, and should not be taken as, medical advice, diagnosis or treatment, and does not replace advice from your GP, fertility specialist or other qualified healthcare provider. Always seek the guidance of a qualified health professional before making changes to your healthcare, treatment or medication, and before starting, stopping or combining any supplements. Individual results can vary and no outcome, including fertility or pregnancy outcomes, is guaranteed.
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