Could your thyroid be a missing piece of your fertility puzzle?
When you’re trying to conceive, it’s easy to focus on the more obvious markers of fertility: your menstrual cycle, whether you’re ovulating, your AMH, egg quality and reproductive hormones.
But there’s another important piece of the puzzle that can sometimes be overlooked: your thyroid.
Your thyroid may be small, but the hormones it produces have far-reaching effects throughout the body, influencing everything from metabolism and energy to menstrual cycles and reproductive health.
For some women, thyroid dysfunction is already diagnosed. For others, symptoms may be subtle, or thyroid health simply hasn’t been explored in much detail.
So, what does your thyroid have to do with fertility — and when might it be worth investigating further?
What does your thyroid actually do?
The thyroid is a small, butterfly-shaped gland situated at the front of your neck.
It produces thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), which help regulate metabolism and influence numerous processes throughout the body.
Your thyroid is part of a carefully regulated feedback system involving the brain and pituitary gland. The pituitary produces thyroid-stimulating hormone (TSH), which signals to your thyroid to produce thyroid hormones.
This is why TSH is usually the first marker checked when thyroid function is investigated.
But thyroid health is more nuanced than simply labelling a TSH result as “normal” or “abnormal”.
Your symptoms, medical history, medications, thyroid hormone levels and, in some circumstances, thyroid antibodies may all help build a more complete picture.
And when you’re preparing for pregnancy, that picture can be particularly important.
Can thyroid problems affect fertility?
Yes. Significant thyroid dysfunction can affect reproductive health.
An underactive thyroid — hypothyroidism — can interfere with normal reproductive function and may contribute to menstrual irregularities and problems with ovulation.
Symptoms of hypothyroidism can include:
- Fatigue
- Feeling unusually cold
- Constipation
- Dry skin
- Hair changes or hair loss
- Unexplained weight changes
- Low mood
- Heavy or irregular periods
- Changes in ovulation
An overactive thyroid — hyperthyroidism — can also disrupt menstrual cycles and reproductive function.
Of course, many of these symptoms are non-specific. Feeling tired or struggling with your weight doesn’t automatically mean you have a thyroid problem.
But if you’re trying to conceive and experiencing symptoms that could suggest thyroid dysfunction, it’s certainly worth discussing appropriate testing with your GP or fertility team.
What should your TSH be when trying to conceive?
This is probably one of the most confusing areas of thyroid and fertility health.
If you’ve spent any time researching fertility online, you may have come across the idea that your TSH must be below 2.5 mIU/L before trying to conceive.
The reality is more nuanced.
Historically, a TSH below 2.5 has often been discussed as an “optimal” fertility target, and some fertility clinics still use this threshold in particular circumstances.
However, more recent evidence does not support treating 2.5 mIU/L as a universal cut-off for every woman trying to conceive.
Current guidance from the American Society for Reproductive Medicine (ASRM), for example, concludes that TSH levels between 2.5 and 4.0 mIU/L have not been shown to increase miscarriage risk. It also recommends using appropriate laboratory reference ranges when assessing thyroid function in non-pregnant women rather than assuming that everybody above 2.5 has subclinical hypothyroidism.
This is an important distinction.
A TSH of 3, for example, doesn’t automatically mean that your thyroid is preventing you from becoming pregnant — nor does it automatically mean that you need thyroid medication.
Equally, a single number shouldn’t necessarily be viewed in isolation.
Your results need to be interpreted within the context of you: your symptoms, medical history, previous thyroid results, whether you already have thyroid disease, your fertility history and any other relevant clinical factors.
This is exactly why I’m such an advocate for testing rather than guessing.
Is testing TSH alone enough?
TSH is usually the starting point when assessing thyroid function and, for many people, it may be all that is initially required.
However, depending on your symptoms, medical history and initial results, your doctor may decide that further investigation is appropriate.
This might include:
Free T4 (FT4)
T4 is the main hormone produced by the thyroid gland. Free T4 measures the proportion circulating in the blood that isn’t bound to proteins and is available to the body’s tissues.
Looking at FT4 alongside TSH can provide additional information about how well the thyroid is functioning.
Free T3 (FT3)
T3 is the more biologically active thyroid hormone.
FT3 isn’t routinely required when screening for hypothyroidism, but there are circumstances in which your medical team may include it as part of a wider thyroid assessment.
Thyroid peroxidase antibodies (TPOAb)
Thyroid peroxidase antibodies are associated with autoimmune thyroid disease, particularly Hashimoto’s thyroiditis.
Thyroglobulin antibodies (TgAb)
These are another type of thyroid antibody that can be associated with autoimmune thyroid disease.
Importantly, this doesn’t mean that every woman trying to conceive needs a comprehensive thyroid panel or thyroid antibody testing.
Testing should be individualised according to your history, symptoms and clinical circumstances.
What are thyroid antibodies, and what do they mean for fertility?
This is another area where there can be a lot of fear and misinformation online.
Hashimoto’s thyroiditis is an autoimmune condition in which the immune system produces antibodies associated with the thyroid. Over time, this can impair the thyroid’s ability to produce sufficient thyroid hormone.
Interestingly, thyroid antibodies can sometimes be detected even when thyroid hormone levels remain within the normal range.
Research has explored whether thyroid autoimmunity itself is associated with infertility or miscarriage, but the evidence is not straightforward.
The presence of thyroid antibodies does not automatically mean that you will struggle to conceive or experience pregnancy loss, and current evidence doesn’t support routinely screening every asymptomatic woman with infertility for thyroid antibodies.
However, targeted testing may sometimes be appropriate — for example, where there is a relevant personal or family history, abnormal thyroid results, symptoms suggestive of thyroid disease or certain fertility histories.
If thyroid antibodies are identified, they should be interpreted alongside thyroid function and your wider clinical picture rather than viewed as an isolated fertility diagnosis.
Why thyroid health becomes even more important during pregnancy
Thyroid hormones are particularly important during pregnancy.
During the early stages of pregnancy, your baby is dependent on maternal thyroid hormone, and pregnancy itself places increased demands on the thyroid.
This is particularly important for women who already have diagnosed thyroid disease.
If you take thyroid medication and are planning a pregnancy, it’s important to discuss this with your GP, endocrinologist or fertility team so that your thyroid function can be appropriately monitored before and during pregnancy.
Don’t stop, start or change thyroid medication or doses without medical guidance.
Nutrition and thyroid health
Nutrition cannot replace appropriate medical treatment for thyroid disease.
However, several nutrients are required for normal thyroid hormone production and metabolism, which means nutritional status remains an important part of the wider picture.
Iodine
Iodine is required to make thyroid hormones.
Requirements increase during pregnancy, making adequate iodine intake particularly important during preconception and pregnancy.
But this is a nutrient where more is definitely not better.
Excessive iodine intake can also interfere with thyroid function, particularly in susceptible individuals. This is why I don’t recommend taking high-dose iodine supplements simply because you’re trying to conceive.
It’s also worth remembering that iodine intake can vary considerably depending on your diet.
Selenium
Selenium contributes to normal thyroid function and helps protect cells from oxidative stress.
Food sources include eggs, meat, fish and Brazil nuts, although selenium content can vary depending on where foods are grown or produced.
Again, selenium is not something I recommend supplementing at high doses without considering your individual requirements and total intake.
Iron
Iron is another nutrient involved in normal thyroid hormone production.
Iron deficiency is also relatively common amongst menstruating women, particularly those with heavy periods, so iron status is something I frequently consider when working with women during preconception.
Zinc
Zinc plays a role in numerous processes throughout the body, including normal fertility and reproduction, immune function and thyroid hormone metabolism.
Good dietary sources include meat, shellfish, dairy products, eggs, nuts, seeds and legumes.
Supplements aren’t always the answer
One thing I see frequently in fertility nutrition is the temptation to add supplement after supplement in an attempt to “optimise” every pathway.
But thyroid health is a perfect example of why this approach isn’t always helpful.
Iodine and selenium are essential nutrients — but excessive amounts can be problematic.
If you already take a prenatal multivitamin alongside separate fertility or thyroid supplements, you may also unknowingly be doubling up on certain nutrients.
The aim isn’t to take as many supplements as possible.
It’s to understand what your body actually needs and build a targeted plan around that.
Don’t forget the bigger fertility picture
If you’ve been trying to conceive for some time, it’s completely understandable to want to find the thing that’s stopping you from getting pregnant.
Maybe it’s your thyroid.
Maybe it’s your progesterone.
Maybe it’s your egg quality.
Maybe it’s your blood sugar.
But fertility rarely comes down to one isolated marker.
Thyroid health is one piece of a much larger picture that may include:
- Ovulation and menstrual health
- Egg and sperm health
- Nutrient status
- Blood sugar regulation
- PCOS
- Endometriosis
- Male-factor fertility
- Sleep
- Stress
- Lifestyle factors
- Age
- Underlying medical conditions
And sometimes, there isn’t one obvious explanation.
This is why my approach to fertility nutrition is highly individualised.
Rather than giving every client the same list of supplements or chasing a supposedly “perfect” laboratory number, I look at the whole picture — your diet, lifestyle, symptoms, medical and fertility history and existing test results — to identify where additional investigation or nutritional support may genuinely be useful.
Trying to conceive and not sure what to test?
Knowing which fertility tests to ask for can feel overwhelming, particularly when you’re receiving different advice from your GP, fertility clinic, friends and the internet.
I’ve created my Essential Fertility Testing Checklist to help make that process a little clearer.
It covers some of the key blood tests and fertility investigations that may be worth discussing with your healthcare team when preparing for pregnancy or investigating fertility difficulties, for both women and their partners.
Download my free Essential Fertility Testing Checklist here.
And if you’d like personalised support, I offer a complimentary discovery call where we can discuss your fertility journey, what support you’re looking for and whether working together feels like the right fit.
Book your complimentary discovery call here.
This article is for educational purposes only and does not constitute medical advice, diagnosis or treatment. Thyroid disorders require appropriate medical assessment and management. If you have concerns about your thyroid function, are taking thyroid medication, or are pregnant or trying to conceive with a diagnosed thyroid condition, please speak to your GP, endocrinologist or fertility team.
