Your thyroid is a small butterfly-shaped gland at the base of your neck, and it governs the speed at which essentially everything in your body runs. Including, it turns out, your menstrual cycle, your ovulation and your ability to hold a pregnancy.
Thyroid problems are common in women, they are straightforward to test for, and they are highly treatable. And yet they are frequently missed in the fertility conversation, partly because the symptoms are so easy to attribute to something else, and partly because standard testing in the UK often does not go far enough.
So this article is genuinely practical. What your thyroid does for fertility, the symptoms worth noticing, and exactly what to ask for when you sit down with your GP.
What your thyroid actually does for fertility
Your thyroid produces two main hormones: T4, which is essentially the storage form, and T3, which is the active form your cells actually use. Your body converts T4 into T3 as needed. Above them sits TSH, thyroid stimulating hormone, released by your pituitary gland to tell your thyroid how hard to work.
That relationship is worth understanding because it explains the test results. TSH works in the opposite direction to what you might expect. When your thyroid is underactive and producing too little hormone, your pituitary shouts louder, so TSH goes up. High TSH generally means an underactive thyroid, not an overactive one.
Thyroid hormones interact directly with your reproductive system in several ways. They influence the pituitary signalling that governs ovulation. They affect the development of your uterine lining, which matters for implantation. They interact with sex hormone binding globulin, which changes how much free oestrogen and testosterone are circulating. And they affect prolactin, which when elevated suppresses ovulation.
When thyroid function is off, the practical effects show up as irregular or absent cycles, cycles that are unusually heavy or unusually light, problems with ovulation, a shortened luteal phase, difficulty conceiving despite ovulating, and increased risk of early miscarriage.
Symptoms worth noticing
The frustrating thing about thyroid symptoms is how easily each one is explained away by something else. Tired? You are busy. Gaining weight? You are getting older. Feeling low? It is the time of year. This is precisely why it goes undetected for so long.
An underactive thyroid, hypothyroidism, typically shows up as persistent fatigue that rest does not fix, feeling cold when others are comfortable, weight gain or difficulty losing weight, dry skin and brittle hair, hair thinning particularly at the outer eyebrows, constipation, low mood or brain fog, heavier or more frequent periods, and muscle aches.
An overactive thyroid, hyperthyroidism, looks rather different: unexplained weight loss, feeling hot and sweating more, a racing or irregular heartbeat, anxiety or feeling wired, tremor in the hands, difficulty sleeping, loose stools, and lighter or absent periods.
Hypothyroidism is considerably more common in women, and the most common cause in the UK is Hashimoto’s thyroiditis, an autoimmune condition where the immune system gradually damages the thyroid gland.
If several of these ring true for you, particularly alongside cycle changes or difficulty conceiving, that is a genuinely good reason to ask for testing rather than waiting.
Why it so often gets missed
It is worth understanding why thyroid dysfunction slips through so regularly, because knowing this helps you advocate for yourself.
The symptoms overlap almost entirely with things women are routinely told are normal. Tiredness with a busy life. Weight changes with age. Low mood with stress. Heavy periods with simply having heavy periods. Each symptom individually has a dozen innocent explanations, and it is only the cluster that points anywhere.
There is also a gender pattern worth naming. Thyroid conditions are around eight to ten times more common in women than men, and women’s fatigue in particular is more likely to be attributed to lifestyle or mood rather than investigated. If you have raised these symptoms before and felt dismissed, you are not imagining that pattern.
Then there is the testing itself. A single TSH result within the population reference range closes the conversation in most appointments, even when that number sits at the upper end and even when antibodies were never checked. Nobody is being negligent; it is simply that population screening and preconception optimisation are different questions, and the standard test answers the first one.
And finally, thyroid function genuinely fluctuates. A single test is a snapshot. TSH varies through the day, tending to be higher in the early morning and lower in the afternoon, so even the timing of your blood draw affects the number you get. If your result is borderline, asking to repeat it, ideally at a consistent time of day, is entirely reasonable.
The connection to other fertility factors
Thyroid problems rarely sit in isolation, which is another reason a full picture matters.
Prolactin. Low thyroid function can raise prolactin, and elevated prolactin suppresses ovulation directly. This is one route by which an underactive thyroid stops periods altogether. If your cycles are absent or very irregular, prolactin is worth testing alongside thyroid.
PCOS. Thyroid conditions and PCOS occur together more often than chance would suggest, and both affect ovulation. Having one does not rule out the other, and where symptoms persist after treating one, the other is worth considering.
Iron. This one runs in both directions. Low iron impairs thyroid hormone production, and an underactive thyroid causes heavier periods, which depletes iron further. It is a genuine loop, and treating only one side often leaves the picture incomplete.
Autoimmunity more broadly. Hashimoto’s frequently travels with other autoimmune conditions, particularly coeliac disease. Coeliac is itself associated with fertility difficulties and is underdiagnosed, so if you have thyroid antibodies and digestive symptoms, it is worth mentioning.
Blood sugar and insulin. Thyroid hormones govern the rate at which your cells use glucose, so an underactive thyroid slows glucose metabolism and can worsen blood sugar instability, which then feeds back into the wider hormonal picture.
Exactly what to ask your GP for
This is the part that matters most, because standard NHS thyroid testing typically includes only TSH, and sometimes T4. That is often adequate for general screening, but when you are trying to conceive, it can miss things.
Here is what is worth asking for, and why.
TSH. The starting point, and the one you will almost certainly be offered. Ask for the actual number rather than being told it is normal, and ask what the lab’s reference range is, because this matters enormously for the next section.
Free T4. Measures the available storage hormone. Usually included if TSH comes back abnormal, but worth requesting alongside from the start.
Free T3. The active hormone your cells actually use. This is frequently not tested on the NHS, and it can be genuinely informative. Some people convert T4 to T3 poorly, so T4 can look fine while the active hormone sits low.
Thyroid antibodies, specifically anti-TPO and anti-thyroglobulin. This is the one I would push hardest for. These detect autoimmune thyroid disease, and here is why it matters: thyroid antibodies are independently associated with fertility difficulties and miscarriage even when TSH and T4 are entirely normal. Research shows antibody-positive women take longer to conceive naturally and have lower IVF success rates. If you only test TSH, you can be told everything is fine while missing this entirely.
It is reasonable to say to your GP: “I am trying to conceive and I would like a full thyroid panel including free T3 and thyroid antibodies, because antibodies are associated with miscarriage risk independently of TSH.” That framing gives a clinical reason rather than sounding like a shopping list.
The number that causes the most confusion
Here is something worth understanding clearly, because it explains why women are so often told their thyroid is fine when it may not be optimal for conception.
Most UK lab reference ranges for TSH go up to around 4.0 to 4.5 mIU/L. If your result is 3.8, you will be told it is normal, and by the general population standard, it is.
However, most fertility specialists work to a tighter target when someone is trying to conceive, generally aiming for a TSH between roughly 1.0 and 2.5 mIU/L. There is reasonable evidence that TSH above 2.5 is associated with reduced fertility in some women, higher miscarriage risk, and poorer IVF outcomes.
I want to be balanced here, because this is genuinely debated. Randomised trials have not consistently shown benefit from treating mildly raised TSH in everyone, and subclinical hypothyroidism resolves on its own without treatment in a meaningful proportion of cases. So this is not a case of everyone with a TSH of 3 needing medication.
But it is absolutely a reason to know your actual number, to ask what it means in the context of trying to conceive specifically, and to ask whether it is worth rechecking or monitoring rather than simply filing under normal. Treatment is more clearly indicated where TSH is genuinely high, where antibodies are present alongside raised TSH, or where you are already pregnant.
Why it matters more once you are pregnant
Your thyroid demand increases substantially in pregnancy, particularly in the first trimester, when your baby is entirely dependent on your thyroid hormone for their developing brain and nervous system. Their own thyroid does not start functioning until around week twelve.
Untreated hypothyroidism in pregnancy is associated with increased risk of miscarriage, preterm birth and effects on the baby’s development. A meta-analysis of eighteen cohort studies found subclinical hypothyroidism associated with roughly double the risk of pregnancy loss.
This is exactly why sorting your thyroid out before conceiving, rather than after, is so worthwhile. If you are already on thyroid medication, your dose usually needs increasing early in pregnancy, often by around a quarter to a third, so do contact your GP as soon as you get a positive test rather than waiting for your booking appointment.
Nourishing your thyroid
Food does not replace treatment where treatment is needed, but a few nutrients genuinely matter for thyroid function.
Iodine is the raw material your thyroid uses to make its hormones, and requirements increase in pregnancy. Good sources include dairy, eggs and white fish. A word of caution though: seaweed and kelp supplements can contain wildly variable and sometimes excessive iodine, which can itself disrupt thyroid function. More is genuinely not better here.
Selenium is needed to convert T4 into active T3, and it also plays a role in reducing thyroid antibodies. Two or three Brazil nuts daily provides plenty. Again, do not overdo it, as selenium has a relatively narrow safe range.
Iron matters because the enzyme that makes thyroid hormone depends on it, and low iron impairs thyroid function. Worth asking for a ferritin test rather than just a full blood count, since ferritin shows your stores.
Zinc is involved in T3 production, and comes readily from pumpkin seeds, shellfish, chickpeas and meat.
Vitamin D is worth testing given its role in immune regulation, particularly relevant with autoimmune thyroid disease.
One practical note if you take levothyroxine: it should be taken on an empty stomach, and minerals including iron and calcium interfere with its absorption, so leave at least four hours between them.
If you are diagnosed
Please know this is genuinely good news rather than bad. A thyroid problem found is a thyroid problem that can be treated, usually with a simple daily tablet, and treatment frequently restores ovulation and improves fertility outcomes considerably. One study found around three quarters of women treated for overt and subclinical hypothyroidism went on to conceive.
Levothyroxine is safe in pregnancy and is one of the more straightforward interventions in the whole fertility landscape. If you are diagnosed, expect regular monitoring, particularly when trying to conceive and through pregnancy, since your requirements change.
What to expect practically. Levothyroxine is taken once daily, ideally first thing on an empty stomach, thirty to sixty minutes before food. Absorption is genuinely affected by other things, so leave four hours between it and any iron or calcium supplement, and be aware that coffee and high-fibre breakfasts can reduce uptake if taken too close together.
Your dose is usually adjusted gradually, with blood tests around every six to eight weeks until levels settle. It can take a few months to find the right dose, which is frustrating when you are keen to get on with trying, but getting it right matters more than getting it fast.
If you become pregnant on levothyroxine, contact your GP as soon as you have a positive test. Requirements typically increase by around a quarter to a third early in pregnancy, and this adjustment usually needs to happen quickly rather than waiting for your booking appointment. Many clinicians will advise increasing your dose immediately on a positive test and testing shortly after. Ask your GP in advance what their plan would be, so you know rather than scrambling.
If your antibodies are positive but TSH is normal, this is a genuinely grey area. Some evidence suggests treating antibody-positive women with levothyroxine improves outcomes, but this remains debated and trials have not consistently shown benefit. What is not debated is that you should be monitored more closely, since antibody-positive women are considerably more likely to develop hypothyroidism over time, and particularly during and after pregnancy. Ask about a monitoring plan rather than being discharged.
Postpartum thyroiditis is worth knowing about too. It affects a meaningful proportion of women in the year after birth, particularly those with thyroid antibodies, and typically involves a temporary overactive phase followed by an underactive one. Because the symptoms overlap so heavily with normal new motherhood, exhaustion, mood changes, hair loss, it is frequently missed. If you have thyroid antibodies, ask about postnatal monitoring before you give birth.
Where to leave this
If you are struggling to conceive, or your cycles have changed, or you simply recognise a cluster of those symptoms, ask for your thyroid to be checked properly. Ask for the numbers, not just the word “normal”. Ask specifically for antibodies. And ask what your result means in the context of trying to conceive, rather than in the context of the general population.
It is one blood test. It might be entirely clear, which is useful information in itself. And occasionally it turns out to be the missing piece nobody had looked for.
A gentle note
This article is for gentle, supportive education and is not medical advice. Please discuss any thyroid concerns and testing with your GP, who can interpret results in the context of your full history. Never start or adjust thyroid medication without medical supervision. If you have been trying to conceive for over a year, or over six months if you are 36 or older, please ask about investigation.
If you would love help making sense of your results and what to do next, I am here. I offer warm, one-to-one fertility coaching alongside your medical care, never instead of it.
Written by
Brydee Thomas
Holistic health coach and founder of Radiant Bloom, supporting women through trying to conceive, pregnancy and the postpartum months. Evidence-based guidance, no programmes and no generic plans.