Thyroid Health and Fertility After 35: What Women Should Know

The thyroid gland plays a central role in reproductive health that is often overlooked in fertility conversations. Thyroid disorders are among the most common endocrine conditions in women, and their prevalence increases with age. For women over 35 who are trying to conceive, understanding the relationship between thyroid function and fertility — and knowing when to get tested — is a practical and important step.

This article provides a research-informed overview of how thyroid function affects fertility, what testing involves, and what women with thyroid conditions should understand when planning pregnancy. As always, specific medical decisions should be made with a healthcare provider’s guidance.

How Thyroid Function Affects Fertility

The thyroid produces hormones (primarily T3 and T4) that regulate metabolism, body temperature, and a wide range of physiological processes — including the hormonal cascade that governs the menstrual cycle. According to the National Institute of Child Health and Human Development, thyroid dysfunction can disrupt ovulation, affect implantation, and increase the risk of miscarriage. Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can impair fertility, though the mechanisms differ.

Hypothyroidism is the more commonly encountered form in the context of female fertility. Even subclinical hypothyroidism — where TSH (thyroid-stimulating hormone) is mildly elevated but T3 and T4 are within normal range — has been associated in some research with reduced fertility and higher miscarriage risk, though findings across studies are not entirely consistent. The clinical threshold at which treatment is recommended in the preconception context is an area of active discussion among endocrinologists and reproductive specialists.

Why This Is Particularly Relevant After 35

Thyroid disorders become more prevalent with age in women. Autoimmune thyroid disease — primarily Hashimoto’s thyroiditis, which causes hypothyroidism, and Graves’ disease, which causes hyperthyroidism — is more common in women generally and increases with advancing age. Women who are already managing other age-related hormonal changes may find that thyroid dysfunction compounds existing fertility challenges in ways that are difficult to disentangle without testing.

Additionally, the presence of thyroid antibodies (anti-TPO antibodies) — even in women with normal TSH levels — has been associated with higher miscarriage rates and lower IVF success rates in some research. This is relevant for women over 35 who have had unexplained pregnancy losses or difficulty conceiving despite normal standard fertility investigations. For the broader context of fertility assessments, what fertility evaluations involve after 35 covers the range of testing that may be recommended.

Thyroid Testing in the Fertility Context

A TSH blood test is the standard first-line test for thyroid function and is straightforward and widely available. Many fertility specialists and some GPs include TSH testing as part of preconception or early fertility workup, particularly for women over 35. If TSH is abnormal, further testing (including free T4 and thyroid antibodies) is typically ordered to clarify the nature and cause of any dysfunction.

Guidelines on the optimal TSH range for women trying to conceive and during pregnancy vary between professional bodies. Most endocrinologists aim for TSH levels below 2.5 mIU/L in women actively trying to conceive with known thyroid disease, though some guidelines differ. If you are trying to conceive and have not had your thyroid tested recently, asking your GP or OB/GYN to include this in your workup is a reasonable request. For the context of how this fits into broader lifestyle and health considerations, lifestyle factors affecting fertility after 35 covers related ground.

Thyroid Conditions and Pregnancy

Thyroid hormone requirements increase during pregnancy — typically by 25–50% — which means women with pre-existing hypothyroidism may need their medication dose adjusted early in pregnancy. This is another reason why women with known thyroid conditions are advised to discuss their medication and monitoring plan with their healthcare provider before conception if possible. Uncontrolled hypothyroidism during pregnancy is associated with adverse outcomes including miscarriage, preterm birth, and neurodevelopmental differences in the child; however, well-managed thyroid disease in pregnancy is associated with normal outcomes for most women.

Frequently Asked Questions

Should I get my thyroid tested before trying to conceive?

For women over 35 who are planning pregnancy, including thyroid function (TSH) in a preconception check is generally considered reasonable, particularly if there is a family history of thyroid disease, previous pregnancy losses, irregular cycles, or symptoms suggesting thyroid dysfunction (unexplained weight changes, fatigue, hair loss, or feeling unusually cold or hot). Your GP or OB/GYN can advise on whether this is appropriate for your situation.

Can I conceive naturally if I have hypothyroidism?

Many women with hypothyroidism conceive naturally, particularly when their thyroid function is well-controlled with medication. Untreated or undertreated hypothyroidism is more likely to affect fertility. If you have a known thyroid condition and are trying to conceive, ensuring your thyroid levels are optimised — ideally before conception — is an important preparatory step.

Does having thyroid antibodies mean I have thyroid disease?

Not necessarily in terms of current function. Many women have elevated thyroid antibodies (anti-TPO or anti-thyroglobulin) with normal TSH and thyroid hormone levels — this is sometimes called euthyroid autoimmune thyroiditis. While these women do not have active thyroid disease, some research suggests this antibody profile is associated with higher miscarriage risk and warrants monitoring during pregnancy. This is an area where specialist endocrine input alongside obstetric care can be valuable.

Key Takeaways

Thyroid disorders are common in women and increase in prevalence with age — they can affect ovulation, fertility, and pregnancy outcomes.
TSH testing is a simple, accessible first step and is often included in preconception or fertility workups for women over 35.
Even subclinical thyroid dysfunction may be relevant in the fertility and pregnancy context — specialist guidance is valuable if thyroid abnormalities are identified.
Women with known thyroid conditions should ideally optimise their thyroid function before conception and plan for increased monitoring during pregnancy.
Well-managed thyroid disease in pregnancy is associated with normal outcomes for most women.

This content is for informational purposes only and does not constitute medical advice. Individual health situations vary significantly. Always consult a qualified healthcare provider before making decisions related to your health, fertility, or pregnancy.

About the Author

Emily Carter is a women’s health writer focused on fertility, pregnancy after 35, and sleep changes in midlife. She writes research-informed, non-alarmist content to help women navigate reproductive and hormonal transitions with clarity and confidence.