The thyroid — a small, butterfly-shaped gland in the neck — has outsized influence on the body’s overall hormonal environment. For women exploring fertility after 35, thyroid health is an area that often comes up in conversations with healthcare providers, and for good reason. Research indicates that thyroid function is closely intertwined with reproductive health, and that thyroid disorders — which are more prevalent in women than in men and tend to become more common with age — can affect various aspects of the menstrual cycle, ovulation, and pregnancy.
This doesn’t mean that thyroid problems are inevitable or that a thyroid issue is necessarily behind any particular fertility challenge. What it does mean is that understanding this connection, and knowing when it might be worth discussing thyroid testing with your doctor, can be genuinely useful background knowledge.
What the Research Indicates About Thyroid and Fertility
According to research cited through the National Institutes of Health, thyroid disorders — including both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) — have been associated with menstrual irregularities, ovulatory dysfunction, and in some cases, difficulties with conception. Thyroid hormones play roles in regulating the menstrual cycle through their interactions with reproductive hormones, and disruptions in thyroid function can affect the hormonal balance needed for regular, ovulatory cycles.
It’s worth emphasizing that these are associations observed in research, not certainties for any individual. Many women with thyroid conditions conceive without difficulty, particularly when the condition is identified and managed appropriately. The key takeaway from the research is that thyroid health is worth including in a comprehensive fertility evaluation, rather than something to assume is fine without testing.
Hypothyroidism: When the Thyroid Is Underactive
Hypothyroidism — the most common thyroid disorder in women of reproductive age — occurs when the thyroid produces insufficient thyroid hormone. Symptoms can be subtle and easy to attribute to other causes: fatigue, difficulty concentrating, feeling cold more easily, weight changes, and changes in mood. In some women, irregular or heavier menstrual cycles are also associated with untreated hypothyroidism.
Subclinical Hypothyroidism and Pregnancy
A category that often comes up in fertility and pregnancy discussions is subclinical hypothyroidism — a state in which TSH (thyroid-stimulating hormone) is elevated but thyroid hormone levels remain within normal range. Research on whether subclinical hypothyroidism affects fertility and pregnancy outcomes has produced mixed results, and clinical guidance in this area has evolved over time. Some studies have suggested associations with increased risk of miscarriage; others have found more limited effects. Current guidelines from major endocrinology and obstetrics organizations address this nuance, and your healthcare provider can help you understand what the research means for your specific TSH level and circumstances.
Hashimoto’s Thyroiditis and Autoimmune Considerations
Hashimoto’s thyroiditis is an autoimmune condition in which the immune system attacks the thyroid gland, gradually reducing its function. It is the most common cause of hypothyroidism in developed countries and is significantly more prevalent in women. For women over 35, the cumulative likelihood of having developed thyroid antibodies increases compared to younger age groups.
Research has explored associations between thyroid autoimmunity (the presence of thyroid antibodies, even in women with normal thyroid function) and reproductive outcomes including miscarriage risk. Some studies suggest that thyroid antibody positivity may be associated with higher rates of pregnancy loss, though the mechanisms are not fully understood and findings are not uniform across research. If you have a history of miscarriage or unexplained fertility challenges, thyroid antibody testing may be part of a comprehensive evaluation — something your reproductive endocrinologist or OB/GYN can advise on.
Hyperthyroidism: When the Thyroid Is Overactive
Hyperthyroidism — less common than hypothyroidism but also more prevalent in women than men — involves excessive thyroid hormone production. Symptoms may include anxiety, heart palpitations, heat intolerance, weight loss, and in some cases, irregular periods. Untreated hyperthyroidism has been associated with ovulatory disruption and is generally managed before or during pregnancy due to potential risks to fetal development. If you have symptoms that might suggest hyperthyroidism, discussing them with your healthcare provider promptly is advisable.
What Thyroid Testing Looks Like
A basic thyroid evaluation typically begins with a blood test measuring TSH. If TSH is outside the normal range, follow-up tests measuring free T4 (and sometimes free T3) help clarify the nature and degree of any dysfunction. Thyroid antibody tests — specifically TPO (thyroid peroxidase) antibodies and TgAb (thyroglobulin antibodies) — can identify autoimmune thyroid disease even when hormone levels are in the normal range.
Many clinicians include TSH as part of a preconception workup for women over 35, and it is a standard component of prenatal lab panels during early pregnancy. If you’re planning to conceive and haven’t had thyroid testing recently, asking your doctor whether it makes sense to check yours is a reasonable step. Understanding what to discuss at preconception health visits after 35 can help you feel prepared for those conversations.
Thyroid Management During Pregnancy
For women with known thyroid conditions, pregnancy introduces additional considerations. Thyroid hormone requirements change during pregnancy — often increasing — and medication doses may need adjustment. According to ACOG and the American Thyroid Association, women taking levothyroxine for hypothyroidism should have their TSH checked in early pregnancy and throughout, with dose adjustments made as needed. This is a well-established aspect of pregnancy management for women with thyroid disease, and most women with treated thyroid conditions have healthy pregnancies.
Frequently Asked Questions
Should every woman over 35 be tested for thyroid disorders before trying to conceive?
Opinions vary among professional organizations — some recommend universal screening for thyroid conditions preconception, while others recommend targeted screening based on symptoms or risk factors. Your OB/GYN or primary care provider can help you decide whether and when testing makes sense given your personal and family health history, any symptoms you’re experiencing, and your overall health picture.
Can thyroid treatment improve fertility outcomes?
For women with overt (not just subclinical) hypothyroidism, treating the condition to bring TSH into a healthy range is generally considered important for both fertility and pregnancy. For subclinical hypothyroidism or thyroid antibody positivity alone, research on whether treatment improves fertility or reduces miscarriage risk is less definitive, and clinical approaches vary. This is an area where working with a reproductive endocrinologist who is current on the research can be particularly valuable.
What TSH level is considered optimal for pregnancy?
Guidelines have evolved on this question. Many endocrinology and obstetrics organizations currently recommend that TSH be kept below 2.5 mIU/L during the first trimester and potentially below 3.0 mIU/L in subsequent trimesters, though specific targets may be individualized. If you’re pregnant or planning to conceive, discussing your specific TSH target with your healthcare provider ensures you’re working from current guidance applied to your situation.
Key Takeaways
- Thyroid disorders are more common in women than men and tend to increase in prevalence with age, making thyroid health a relevant consideration for women navigating fertility after 35.
- Research links both hypothyroidism and hyperthyroidism to menstrual irregularities and potential challenges with ovulation, though many women with thyroid conditions conceive without difficulty when conditions are properly managed.
- Hashimoto’s thyroiditis and thyroid antibody positivity have been associated in some studies with higher miscarriage risk, though findings are mixed and clinical guidance continues to evolve.
- TSH is a standard component of many preconception and early pregnancy evaluations; asking your doctor about thyroid testing is a reasonable step if you haven’t been screened recently.
- Women with known thyroid conditions generally require closer monitoring and possible medication adjustments during pregnancy — something your healthcare provider will guide you through.
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.