The thyroid gland may be small, but its influence on reproductive health is substantial. For women 35 and older who are trying to conceive or who are pregnant, thyroid function is a topic that their healthcare providers may raise — and one that’s worth understanding. Yet thyroid health and fertility is an area where misinformation is common, making evidence-based context particularly valuable.
This article explores what research tells us about the relationship between thyroid function and reproductive health after 35, including what types of thyroid issues are most relevant, how they may affect fertility and pregnancy, and when thyroid testing makes sense. All of this information is intended as educational background; individual situations vary considerably, and any concerns about thyroid health are best addressed with a qualified healthcare provider.
Understanding Thyroid Function Basics
The thyroid gland, located in the neck, produces hormones (primarily thyroxine, or T4, and triiodothyronine, or T3) that regulate metabolism and many other bodily functions. Thyroid-stimulating hormone (TSH), produced by the pituitary gland, is the marker most commonly used to assess thyroid function in clinical practice — though it’s one piece of a broader picture.
Thyroid disorders are significantly more common in women than in men, and prevalence tends to increase with age. According to research referenced by the National Institutes of Health, an estimated 1 in 8 women will develop a thyroid condition at some point in their lives. For women over 35 who are trying to conceive or navigating pregnancy, understanding thyroid health is therefore particularly relevant.
The two primary categories of thyroid dysfunction — hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) — both have implications for reproductive health, though hypothyroidism is more commonly discussed in the context of fertility and pregnancy.
Hypothyroidism and Fertility: What the Research Shows
Hypothyroidism occurs when the thyroid gland doesn’t produce enough thyroid hormone. It can range from overt (clearly abnormal TSH with symptoms) to subclinical (mildly elevated TSH without obvious symptoms). Both forms have been studied in the context of fertility and pregnancy.
Effects on Ovulation and Cycle Regularity
Research has established that overt hypothyroidism can disrupt menstrual cycle regularity and ovulation. Thyroid hormones interact with sex hormone pathways, and when thyroid function is significantly impaired, these interactions can affect the hormonal environment necessary for regular ovulation. More subtle subclinical hypothyroidism has a less clear-cut relationship with fertility, and research findings are mixed — it remains an area of ongoing clinical study.
Thyroid Antibodies and Fertility
Autoimmune thyroid conditions — particularly Hashimoto’s thyroiditis, the most common cause of hypothyroidism in developed countries — are associated with the presence of thyroid antibodies (anti-TPO and anti-thyroglobulin antibodies). Some research suggests that the presence of these antibodies, even in women with normal TSH levels, may be associated with slightly increased risk of miscarriage and some fertility challenges, though the mechanisms and clinical significance are still being studied. For women navigating the process of trying to conceive after 35, thyroid antibody testing is sometimes recommended by reproductive endocrinologists.
Thyroid Health During Pregnancy
Pregnancy places significant demands on the thyroid. Thyroid hormone requirements increase by roughly 25-50% during pregnancy to support fetal brain and neurological development (the fetus depends entirely on maternal thyroid hormone in the first trimester, before its own thyroid is functional). Women who had adequate thyroid function before pregnancy may develop relative hypothyroidism during pregnancy, and those with pre-existing thyroid conditions often require medication adjustments.
According to the American College of Obstetricians and Gynecologists (ACOG), overt hypothyroidism in pregnancy is associated with increased risks including pregnancy complications and effects on fetal neurodevelopment, and treatment with levothyroxine (synthetic thyroid hormone) is well-established. Subclinical hypothyroidism in pregnancy is an area of ongoing research and clinical debate — guidelines continue to evolve on whether and when to treat.
Postpartum Thyroiditis
Some women develop postpartum thyroiditis — a thyroid inflammation that typically occurs in the first year after delivery. It often follows a pattern of hyperthyroidism (overactivity) followed by hypothyroidism. Symptoms can mimic postpartum depression or general new-parenthood exhaustion, making it easy to overlook. Women with a history of thyroid disease or autoimmune conditions may be at higher risk. For context on postpartum wellbeing after 35, this is a topic worth being aware of.
When to Consider Thyroid Testing
There is ongoing debate among professional organizations about universal vs. targeted thyroid screening before and during pregnancy. Current guidance varies, but many clinicians recommend thyroid screening (at minimum a TSH test) for women with:
- A personal or family history of thyroid disease
- Symptoms suggestive of thyroid dysfunction (unexplained fatigue, weight changes, cold or heat intolerance, hair changes)
- Type 1 diabetes or other autoimmune conditions
- A history of miscarriage or prior pregnancy complications
- Difficulty conceiving after several months of trying
- Neck enlargement or symptoms that might indicate a thyroid issue
For women over 35 who are actively trying to conceive, a conversation with a gynecologist or reproductive endocrinologist about whether thyroid testing is appropriate is a reasonable step. Thyroid testing is a blood test with a low burden, and for women with risk factors, the information can be genuinely useful.
Treatment and Management Considerations
When thyroid dysfunction is identified, treatment approaches depend on the type and severity. Overt hypothyroidism is generally treated with levothyroxine, a well-studied synthetic thyroid hormone that is considered safe during pregnancy. Hyperthyroidism has a more complex management picture, particularly during pregnancy, and typically involves a specialist.
For women with subclinical hypothyroidism or thyroid antibodies, treatment decisions are more nuanced and are the subject of ongoing research and evolving clinical guidelines. Rather than making assumptions about whether treatment is needed, working with a knowledgeable clinician who can review individual lab values, symptoms, and health history is the most reliable path.
Frequently Asked Questions
Can an underactive thyroid cause infertility?
Overt hypothyroidism is associated with menstrual irregularity and disrupted ovulation in some women, which can affect fertility. When adequately treated, thyroid-related fertility challenges often improve. Subclinical hypothyroidism has a less certain relationship with fertility, and the decision to treat is individualized.
Should I get my thyroid checked before trying to conceive?
There’s no universal recommendation for thyroid screening before conception, but many clinicians do recommend testing for women with risk factors (personal or family history of thyroid disease, autoimmune conditions, symptoms, prior miscarriage). A conversation with your healthcare provider can help determine whether testing makes sense for your individual situation.
If I’m on thyroid medication, can I still get pregnant?
Most women with well-controlled thyroid conditions on appropriate medication can conceive and have healthy pregnancies. Thyroid hormone requirements typically increase during pregnancy, so close monitoring and possible dose adjustments are generally needed. Working with both an obstetrician and an endocrinologist during pregnancy is often advisable for women with thyroid conditions.
Does Hashimoto’s thyroiditis affect pregnancy even with normal TSH?
Some research suggests an association between thyroid antibodies and increased miscarriage risk even in women with normal TSH levels, though the clinical significance is debated and the evidence continues to evolve. A reproductive endocrinologist can discuss whether this is relevant to an individual’s situation and what monitoring might be appropriate.
Key Takeaways
- Thyroid dysfunction — particularly hypothyroidism — is more common in women and increases with age, making it a relevant consideration for women over 35 who are trying to conceive or pregnant.
- Overt hypothyroidism is associated with disrupted ovulation and menstrual irregularity; when treated, thyroid-related fertility challenges often improve.
- Pregnancy significantly increases thyroid hormone demands, and women with known thyroid conditions typically need closer monitoring and possible medication adjustments during pregnancy.
- Autoimmune thyroid conditions (Hashimoto’s) and thyroid antibodies are areas of ongoing research in the context of fertility and pregnancy.
- Women with thyroid-related concerns or risk factors should discuss appropriate testing and management with a gynecologist, reproductive endocrinologist, or endocrinologist.
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.