The thyroid gland — a small, butterfly-shaped structure in the neck — produces hormones that regulate metabolism, energy, body temperature, and a wide range of physiological processes. What is less commonly discussed is the significant influence thyroid function has on reproductive health, including menstrual cycle regularity, ovulation, the ability to conceive, and pregnancy outcomes.
Thyroid disorders become more prevalent in women as they age, and the late thirties and forties represent a time of increased risk for both underactive (hypothyroidism) and autoimmune thyroid conditions. For women in this age group who are experiencing fertility challenges, cycle irregularity, or difficulty maintaining early pregnancies, thyroid function is often one of the first areas evaluated by reproductive medicine specialists.
How the Thyroid Affects Reproductive Function
Thyroid hormones interact with the reproductive system at multiple levels. They influence the hypothalamic-pituitary-ovarian axis — the hormonal signaling cascade that regulates ovulation — and play a direct role in the health of the uterine lining. Research has found that both hypothyroidism and hyperthyroidism can disrupt menstrual cycle regularity and ovulation, and that thyroid autoimmunity (the presence of thyroid antibodies, even without overt thyroid dysfunction) is associated with reduced fertility and increased miscarriage risk in some studies.
The relationship between thyroid function and fertility is an area of ongoing research, and the clinical implications continue to be refined. What is well-established is that thyroid disorders are among the more treatable causes of menstrual irregularity and fertility challenges, which is one reason why thyroid screening is a standard component of many fertility evaluations.
Hypothyroidism and Fertility
Hypothyroidism — where the thyroid produces insufficient thyroid hormone — is the more common thyroid disorder in women and becomes increasingly prevalent with age. Symptoms can include fatigue, weight gain, cold intolerance, constipation, dry skin, and depression, though many women with mild hypothyroidism (sometimes called subclinical hypothyroidism) have few or no noticeable symptoms.
According to the National Institute of Child Health and Human Development, thyroid hormone is important for normal ovarian function and the hormonal environment of the uterus during implantation. Hypothyroidism has been associated with anovulation, luteal phase dysfunction, and in some research, with higher rates of miscarriage. Subclinical hypothyroidism during pregnancy — even without overt symptoms — has been associated with adverse pregnancy outcomes in some studies, which is why thyroid function testing early in pregnancy or during preconception evaluation is commonly recommended for women over 35.
Hashimoto’s Thyroiditis
Hashimoto’s thyroiditis is an autoimmune condition in which the immune system gradually damages thyroid tissue, often leading to hypothyroidism over time. It is one of the most common autoimmune conditions in women and becomes more prevalent with age. Research has found that the presence of thyroid antibodies (even in women with currently normal thyroid hormone levels) is associated with increased risk of miscarriage and IVF failure in some studies, though this area continues to be studied. If you have a known history of Hashimoto’s or thyroid antibodies, ensuring your thyroid function is optimally managed during preconception planning and pregnancy is important to discuss with your healthcare provider. For context on how hormonal factors more broadly affect fertility in this age group, our article on fertility after 35: what to know provides a broader overview.
Hyperthyroidism and Fertility
Hyperthyroidism — excess thyroid hormone production — is less common than hypothyroidism but can also affect reproductive health. It may cause irregular periods (sometimes very light or infrequent), difficulty conceiving, and, if uncontrolled during pregnancy, is associated with serious maternal and fetal complications. Graves’ disease, the most common cause of hyperthyroidism, is an autoimmune condition that predominantly affects women and may first become apparent during the reproductive years.
Symptoms of hyperthyroidism can include unintentional weight loss, heart palpitations, heat intolerance, tremor, anxiety, and disrupted sleep — some of which can be confused with other conditions. If you experience these symptoms, thyroid function testing is a straightforward way to investigate whether thyroid overactivity may be involved.
Thyroid Function During Pregnancy
Pregnancy places significant demands on the thyroid gland. Thyroid hormone requirements increase by approximately 30–50% during pregnancy, and the maternal thyroid must compensate for this increased demand. Women with pre-existing hypothyroidism — even those well-controlled before pregnancy — commonly need dose adjustments in early pregnancy. For this reason, thyroid function should be checked promptly once pregnancy is confirmed in women with known thyroid conditions.
Research has also found that iodine — essential for thyroid hormone production — is needed in higher amounts during pregnancy and breastfeeding. Most prenatal vitamins now include iodine, though iodine content varies between products. Our overview of prenatal nutrition after 35 includes a section on iodine requirements that may be useful in this context.
Getting Your Thyroid Tested
Thyroid function is assessed through blood tests. TSH (thyroid-stimulating hormone) is typically the initial screening test — it is sensitive to thyroid dysfunction and provides a useful overview of thyroid status. If TSH is abnormal, additional tests (free T4, free T3, thyroid antibodies) may be performed for a more complete picture.
There is some debate in the medical literature about the optimal TSH range for women trying to conceive and during early pregnancy, with some fertility specialists recommending a TSH target lower than the standard laboratory reference range for this population. This is a nuanced area where individual guidance from a healthcare provider — particularly one familiar with reproductive medicine — is valuable.
Frequently Asked Questions
Should I have my thyroid tested before trying to conceive after 35?
Many reproductive endocrinologists and OB/GYNs include thyroid screening as part of a preconception evaluation, particularly for women over 35 or those with symptoms suggesting thyroid dysfunction. Even without symptoms, thyroid testing before pregnancy allows any issues to be identified and treated before conception, which may reduce associated risks.
If I have hypothyroidism, can I still get pregnant?
Many women with well-managed hypothyroidism conceive without difficulty. The key is ensuring thyroid levels are well-controlled — ideally before conception and throughout pregnancy. If hypothyroidism is a contributing factor to fertility challenges, appropriate treatment may help improve reproductive outcomes. A reproductive endocrinologist can help evaluate your specific situation.
Does thyroid disease affect miscarriage risk?
Some research suggests associations between thyroid autoimmunity and higher miscarriage rates, even in women with currently normal thyroid hormone levels. Whether and how to treat this remains an area of active research and clinical debate. If you have thyroid antibodies and a history of miscarriage, discussing this specifically with a reproductive medicine specialist is appropriate.
Key Takeaways
- Thyroid disorders — including hypothyroidism and autoimmune thyroid conditions — become more prevalent in women as they age and can affect menstrual regularity, ovulation, fertility, and pregnancy outcomes.
- Both overt and subclinical hypothyroidism have been associated with fertility challenges and adverse pregnancy outcomes in research, making thyroid screening a standard part of preconception and fertility evaluation.
- Women with pre-existing thyroid conditions typically need thyroid function reassessment in early pregnancy, as thyroid hormone requirements increase significantly during gestation.
- Thyroid disorders are among the more treatable causes of reproductive challenges — with appropriate management, many women with thyroid conditions achieve healthy pregnancies.
- If you have symptoms suggesting thyroid dysfunction or are planning a pregnancy after 35, thyroid testing is a simple and valuable step to discuss with your healthcare provider.
Medical Disclaimer
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.