Thyroid Problems and Fertility: What the Evidence Shows

The honest picture, with numbers

Thyroid testing has become a routine part of the fertility conversation, often at the patient's request, and the guidance is more restrained than the internet suggests. Two things are true at once: overt thyroid disease genuinely matters for pregnancy, and the case for treating borderline results to improve fertility is weak.

Start with what the guidelines actually say. NICE guideline NG257 recommends offering thyroid function testing to people with possible fertility problems only if they have symptoms of thyroid disease. On subclinical hypothyroidism — a raised TSH with a normal free thyroxine — the NICE committee reviewed the evidence and made a recommendation for research rather than a recommendation for practice. That is guidance language for: we looked, and there was not enough there.

ASRM's 2024 guideline reached a similar conclusion more explicitly. It recommends a TSH upper limit of 4.12 mIU/L for people in iodine-sufficient areas who are not pregnant or are attempting pregnancy, and states that it is not recommended to treat pregnant women or women desiring pregnancy who have a diagnosis of subclinical hypothyroidism with levothyroxine, as treatment has not been demonstrated to reduce pregnancy loss nor to improve clinical pregnancy or live birth outcomes. That recommendation carries a Strength B, moderate-confidence grade.

Cochrane's review of thyroxine replacement in subfertile women puts numbers on it. In euthyroid autoimmune thyroid disease, levothyroxine gave a live birth risk ratio of 1.04 (95% CI 0.83 to 1.29) across two trials and 686 women, low-quality evidence. In subclinical hypothyroidism the risk ratio was 2.13 (95% CI 1.07 to 4.21) — but from a single trial of 64 women, also low quality. The review's own conclusion was that no clear conclusions can be drawn.

What actually affects it

Overt hypothyroidism is a different matter

None of the above applies to diagnosed, treated hypothyroidism. The NHS is clear that if you have an underactive thyroid and are pregnant or planning to get pregnant you should tell your GP, that you will need further tests, and that your levothyroxine dose is likely to need changing during pregnancy. Untreated, it increases the chance of serious complications including pre-eclampsia, birth defects, premature birth and miscarriage. Treatment usually prevents these.

The symptoms that should trigger a test

Because NG257 gates testing on symptoms, it is worth knowing what counts. The NHS lists extreme tiredness, feeling cold more than usual, weight gain, constipation, difficulty concentrating, low mood, dry skin or hair, hair loss, a hoarse voice, and — directly relevant here — irregular or heavy periods. Symptoms usually develop slowly. Say them out loud at the appointment rather than assuming a thyroid test is part of the standard panel, because under NG257 it is not.

Thyroid antibodies

ASRM does not recommend screening for thyroid autoimmunity in asymptomatic women with infertility or pregnancy, though targeted screening may be considered in women with a history of recurrent pregnancy loss. Its stated basis is intermediate-quality and conflicting evidence insufficient to establish an association between thyroid antibodies and miscarriage. This is a Grade C, weak recommendation. A positive TPO antibody result in someone with normal thyroid function is common, frequently found by accident, and frequently over-interpreted.

The 2.5 mIU/L number

You will encounter the claim that TSH must be under 2.5 mIU/L to conceive. That threshold came from earlier guidance and has not held up. ASRM's current position is to use laboratory-specific cutoffs, with 4.12 mIU/L for non-pregnant patients in iodine-sufficient areas, reduced by 0.5 mIU/L in the first trimester. There is evidence that TSH above 4 mIU/L during pregnancy is associated with miscarriage, and insufficient evidence that levels between 2.5 and 4 are. If you have been told your TSH of 3.1 is why you have not conceived, that is a stronger claim than the evidence supports.

Hyperthyroidism

An overactive thyroid also disrupts cycles and needs specialist management before and during pregnancy, partly because some treatments are unsuitable in early pregnancy. It is much less common than hypothyroidism but it is a genuine reason for symptom-triggered testing.

What to do next

1. If you already have thyroid disease, get a pre-pregnancy review. This is the single highest-value action in this article — dose adjustment before and during pregnancy is where the evidence is strongest.
2. List your symptoms explicitly if you want a thyroid test, because NG257 gates testing on symptoms rather than on trying to conceive.
3. Ask what your laboratory's reference range is rather than comparing your result to a number from a forum. ASRM recommends laboratory-specific cutoffs.
4. Do not accept levothyroxine as a fertility treatment for a borderline TSH without a conversation about what the trials showed. ASRM notes treatment carries side effects, risks including misdiagnosis and delayed care, and costs.
5. Do not let a thyroid work-up delay the rest of your investigation. Progesterone timing, tubal assessment and semen analysis should proceed in parallel.
6. If you are on levothyroxine, tell whoever prescribes it that you are trying, so monitoring can be arranged as soon as you conceive rather than at booking.

When to seek help

See a GP promptly, outside any fertility timetable, if you have symptoms of thyroid disease — particularly persistent tiredness with cold intolerance, weight change, or periods that have become irregular or much heavier. Thyroid disease is straightforward to test for and straightforward to treat, and the reason to treat it is your health and the safety of a future pregnancy, not a fertility statistic.

For the fertility timetable itself, NG257 recommends assessment of both partners after 1 year of unprotected vaginal intercourse and referral at first presentation if the woman trying to conceive is 36 or over, or if either partner has a suspected or known clinical cause of infertility. Diagnosed thyroid disease is a known clinical cause and qualifies for earlier referral.

One caution worth carrying into private clinics: thyroid panels, antibody panels and levothyroxine for borderline results are commonly sold as fertility interventions. Ask which guideline recommends the test, and what the trial evidence shows for the outcome you care about.