Thyroid conditions and fertility: how they affect conception and early pregnancy

Trying to conceive · Updated July 2026 · All articles

Why the thyroid matters for reproductive health

The thyroid gland, a small butterfly-shaped gland at the front of the neck, produces hormones (mainly T3 and T4) that regulate metabolism throughout the body. These hormones influence almost every system, including the reproductive system. Thyroid hormones interact with the hormones that control ovulation, and disruption at any point in this chain can affect cycle regularity, egg quality and the uterine environment.

Thyroid conditions are among the most common endocrine disorders in women of reproductive age. Hypothyroidism (underactive thyroid) is far more common than hyperthyroidism (overactive thyroid) in this population. Both can affect fertility, though they do so through different mechanisms.

Because thyroid conditions often develop gradually and can present with symptoms that are easy to attribute to other things (tiredness, weight changes, mood shifts), they are frequently not investigated until a fertility problem prompts blood tests. Knowing the signs and asking for testing early can make a meaningful difference.

Hypothyroidism and its effects on fertility

When the thyroid is underactive, it produces insufficient T3 and T4. The pituitary gland responds by producing more thyroid-stimulating hormone (TSH) in an attempt to drive the thyroid harder, which is why a raised TSH is the key marker of hypothyroidism. Reduced thyroid hormone levels can disrupt the hormonal cascade that triggers ovulation, leading to irregular cycles, anovulatory cycles (where no egg is released) or luteal phase defects.

Hypothyroidism is also associated with higher rates of early miscarriage. Even when conception occurs, insufficient thyroid hormone in early pregnancy can affect implantation and early embryo development. The developing baby is entirely dependent on maternal thyroid hormones during the first trimester before its own thyroid becomes functional.

The good news is that hypothyroidism is highly treatable. Levothyroxine (synthetic T4) restores thyroid hormone levels to normal, and for most people with overt hypothyroidism this significantly improves cycle regularity, reduces miscarriage risk and brings fertility outcomes closer to baseline.

Hyperthyroidism and fertility

An overactive thyroid produces excess thyroid hormone, which can also disrupt the reproductive hormonal axis. Hyperthyroidism is associated with irregular cycles, reduced fertility and higher miscarriage rates, though it is considerably less common than hypothyroidism as a fertility concern.

The most common cause of hyperthyroidism is Graves' disease, an autoimmune condition. Treatment options include antithyroid medications, radioiodine therapy or surgery. Becoming pregnant while hyperthyroidism is uncontrolled carries risks for both mother and baby, so most specialists recommend bringing thyroid levels under control before trying to conceive.

If you are on antithyroid medication and planning a pregnancy, a review with an endocrinologist is important. Some medications used to treat hyperthyroidism carry risks in early pregnancy, and dose adjustments or medication changes may be needed in preparation for conception.

Subclinical hypothyroidism: a grey area

Subclinical hypothyroidism is defined as a raised TSH with normal free T4 levels. The person may have few or no symptoms, but the pituitary is already working harder than usual to compensate for slightly reduced thyroid output. This is a common finding, particularly in women, and the question of whether and how to treat it in the context of fertility is one of the more debated areas in reproductive medicine.

Some studies associate subclinical hypothyroidism with higher miscarriage rates, impaired implantation and reduced IVF success rates. Others show minimal impact on outcomes. The disagreement largely comes down to where you draw the line: the standard laboratory "normal" upper limit for TSH is typically around 4.5 mIU/L, but many fertility specialists recommend aiming for below 2.5 mIU/L when conception is the goal.

Current guidance from many fertility and endocrine organisations supports treating subclinical hypothyroidism with levothyroxine when TSH is above 2.5 to 4 mIU/L and the person is trying to conceive or undergoing fertility treatment. The treatment is straightforward, inexpensive and well-tolerated, which tips the risk-benefit balance towards treatment for most people.

What TSH level should I be aiming for?

This is one of the most common questions people have when thyroid health and fertility intersect. The standard laboratory reference range for TSH is approximately 0.4 to 4.5 mIU/L, meaning values within this range are reported as "normal." However, this range reflects the general population, not the specific needs of someone trying to conceive.

Many reproductive endocrinologists and fertility specialists use a more stringent target of TSH below 2.5 mIU/L for people trying to conceive and below 2.0 mIU/L during the first trimester of pregnancy. This is because the thyroid demands during early pregnancy increase rapidly (by around 30 to 50 percent), and a TSH closer to 3 or 4 mIU/L before conception may tip into overt hypothyroidism once pregnancy is established.

If your TSH is within the standard normal range but above 2.5, it is worth having a specific conversation with your GP or a fertility specialist about whether treatment or closer monitoring is appropriate given your circumstances. Do not feel you need to accept "your levels are normal" as the end of the conversation when fertility is your concern.

Hashimoto's thyroiditis and fertility

Hashimoto's thyroiditis is an autoimmune condition in which the immune system gradually attacks and damages the thyroid gland. It is the most common cause of hypothyroidism and is particularly prevalent in women of reproductive age. It is diagnosed by the presence of elevated thyroid peroxidase (TPO) antibodies, often alongside a raised TSH.

Research suggests that the presence of thyroid antibodies, even when TSH remains within normal range, is associated with a higher risk of miscarriage and implantation failure in IVF cycles. The mechanism may involve a generalised autoimmune activation that also affects the endometrium and early embryo development.

Some studies have shown that treating Hashimoto's with levothyroxine (to suppress TSH to the lower end of normal) may reduce miscarriage rates in people with positive antibodies, though this remains an area of active research. If you have Hashimoto's and are trying to conceive, discussing antibody status and treatment goals with a specialist is worthwhile.

Iodine and thyroid health

Iodine is a key building block for thyroid hormones. Severe iodine deficiency is a leading cause of hypothyroidism globally, though it is less common in countries where dietary iodine is adequate. In the UK, iodine is mainly found in dairy products, eggs and fish. People following a vegan or dairy-free diet may have lower iodine intake.

During pregnancy, iodine requirements increase significantly because the baby depends on maternal thyroid hormones in early gestation before its own thyroid is functional. Most UK health guidance recommends that pregnant people or those planning a pregnancy who follow a plant-based diet take an iodine supplement of around 150 micrograms daily.

It is worth being cautious about supplements derived from kelp or seaweed, which contain highly variable and sometimes very large amounts of iodine. Excessive iodine can actually impair thyroid function, so a standardised supplement formulation is preferable to food-based sources when supplementing specifically for thyroid support.

When to ask for a thyroid test

Thyroid function testing is a simple, inexpensive blood test available through your GP. You do not need to wait for a fertility referral to ask for one. If you are actively trying to conceive, requesting a thyroid function test alongside any other preconception checks is entirely reasonable.

There are certain situations where a thyroid check is especially important before or during trying to conceive: if you have a personal or family history of thyroid disease, if you have had unexplained miscarriages, if your cycles are irregular or you have symptoms of thyroid dysfunction (fatigue, weight changes, feeling unusually cold or warm, hair thinning), or if you have any other autoimmune condition.

Ask specifically for a full thyroid panel including TSH, free T4 and TPO antibodies if thyroid autoimmune disease is possible. A TSH alone may miss early Hashimoto's in someone whose TSH is still within range but antibodies are elevated.

Managing thyroid conditions in early pregnancy

If you are on levothyroxine and become pregnant, your dose will almost certainly need to increase. Thyroid hormone requirements typically rise by 25 to 50 percent in the first trimester, and failing to adjust the dose promptly can lead to transient hypothyroidism at a critical time for foetal brain development. Most endocrinologists and obstetricians recommend increasing the dose by one to two tablets per week (the equivalent of roughly 25mcg extra daily) as soon as pregnancy is confirmed and then reviewing thyroid levels every four to six weeks throughout the first trimester.

If you are taking antithyroid medication for hyperthyroidism, a preconception review with a specialist is important because some medications (particularly carbimazole) carry a risk to the developing foetus and may need to be switched or stopped before pregnancy. Planning this in advance, rather than discovering it after a positive test, avoids avoidable risk.

For people with Hashimoto's or subclinical hypothyroidism who were not previously on medication, early pregnancy is a trigger for close monitoring. Thyroid function should ideally be checked within the first few weeks of a positive pregnancy test, and the threshold for starting or adjusting treatment becomes more conservative once pregnancy is confirmed.

Frequently asked questions

What TSH level is ideal when trying to conceive?

Many fertility specialists recommend aiming for a TSH below 2.5 mIU/L when trying to conceive, as values above this threshold have been associated with higher miscarriage rates and impaired implantation in some studies. The standard laboratory reference range is broader (up to around 4.5 mIU/L), so you may need to specifically ask for a fertility-focused review of your results.

Can hypothyroidism cause miscarriage?

Yes. Untreated or poorly controlled hypothyroidism is associated with higher rates of early miscarriage and pregnancy loss. This is one reason thyroid function is worth checking before conception and monitoring closely in early pregnancy, even in people whose thyroid was previously well managed.

Is Hashimoto's thyroiditis common in people trying to conceive?

Hashimoto's is the most common cause of hypothyroidism in women and is relatively common in people trying to conceive. The presence of thyroid antibodies, even with a normal TSH, is associated with a slightly higher risk of miscarriage and implantation failure, which is why some specialists check for antibodies alongside TSH.

Will treating my thyroid condition improve my fertility?

For overt hypothyroidism, treatment with levothyroxine typically improves cycle regularity and reduces miscarriage risk, which can meaningfully improve fertility outcomes. For subclinical hypothyroidism the evidence is less clear, but many specialists treat it when TSH is above 2.5 mIU/L and conception is the goal.

How is thyroid function checked?

Thyroid function is assessed through a blood test measuring thyroid-stimulating hormone (TSH). A low TSH suggests an overactive thyroid; a high TSH suggests an underactive one. When trying to conceive, it is worth also requesting TPO antibody tests to check for Hashimoto's, as these are not always included in a standard thyroid screen.

Do I need more iodine when trying to conceive?

Iodine is essential for thyroid hormone production and particularly important during pregnancy. Most people in the UK get adequate iodine from dairy, eggs and fish. Those following a vegan diet may be at risk of deficiency and should consider a supplement. Avoid supplements derived from kelp or seaweed, which can deliver unpredictable and sometimes excessive iodine doses.

This article is for general information only. If you have concerns about your fertility or reproductive health, speak to your GP or a fertility specialist.