Subclinical hypothyroidism: a small hormone problem that can have a big impact on fertility

If you're trying to conceive, you may have heard your doctor mention "thyroid function tests" as part of your fertility workup. One condition worth knowing about is subclinical hypothyroidism (SCH) — a mild, often symptom-free under-activity of the thyroid gland that's surprisingly common, affecting around 4-8% of women of reproductive age.

What is it?

Your thyroid produces hormones that regulate metabolism, energy and reproductive function. In SCH, your thyroid-stimulating hormone (TSH) level is slightly raised, but your actual thyroid hormone (FT4) level remains normal. It's essentially your pituitary gland working a little harder to keep things balanced, often without you noticing any symptoms at all.

Why does it matter for fertility and pregnancy?

Thyroid hormone plays a key role in regulating ovulation, so when SCH is present, some women experience irregular cycles or anovulation, which can make conceiving naturally more difficult. SCH also appears to be more common in women experiencing infertility than in those with no fertility problems, with one study finding SCH diagnosed in around 14% of women with infertility compared with under 4% of women with confirmed fertility.

Once pregnant, the picture is more nuanced. Subclinical hypothyroidism has been linked with miscarriage, infertility, and adverse obstetric outcomes in some studies, though the strength of this association varies. Reassuringly, a TSH between 2.5 and 4.0 mIU/L is not thought to increase the risk of miscarriage. However, higher TSH levels picked up in early pregnancy have been associated with complications.

What's clearer is that untreated hypothyroidism, whether overt or subclinical, is associated with adverse pregnancy outcomes including miscarriage, and if SCH progresses to overt hypothyroidism (where FT4 actually drops), this carries a well-established increased risk of infertility, miscarriage and other complications.

Should everyone be screened?

Current guidance doesn't recommend blanket screening for all women trying to conceive. However, targeted testing may be worthwhile for women with a history of irregular cycles, recurrent pregnancy loss, or those with risk factors such as a personal or family history of thyroid disease, or unexplained sub-fertility.

How is it diagnosed and treated?

Diagnosis is via a simple blood test. Importantly, the "normal" TSH range can vary slightly by laboratory and by trimester of pregnancy — a lower threshold is generally applied in the first trimester of pregnancy compared with outside of pregnancy.

A common point of confusion is when treatment should actually start. The general guidance is that thyroxine (levothyroxine) is not routinely started for TSH levels below 4.0 mIU/L, since at this level treatment hasn't been shown to improve outcomes such as miscarriage risk. However, once TSH rises above 4.0 mIU/L, treatment with levothyroxine is generally recommended.

Once a woman is established on levothyroxine, the goal shifts: the dose is then titrated to maintain a TSH below 2.5 mIU/L, which reflects the optimal range for conception and early pregnancy. This is why thyroid function is monitored regularly during treatment and adjustments are often needed, particularly once pregnancy is confirmed, as hormone requirements increase.

The takeaway

Subclinical hypothyroidism is common, usually silent, and its impact on fertility and pregnancy isn't always straightforward — but it's a simple, cheap test that can form an important piece of the fertility puzzle. If you have irregular cycles, a history of pregnancy loss, thyroid problems, or are struggling to conceive, it's worth discussing thyroid testing with your specialist.

This blog is for general information only and does not replace individualised medical advice. If you're experiencing fertility difficulties or have concerns about thyroid health, please speak with your specialist.