Thyroid and Fertility: The Connection Your Doctor Might Miss
Your thyroid is a small butterfly-shaped gland in your neck, and it has an outsized impact on your fertility.
Thyroid dysfunction is one of the most common hormonal issues in women of reproductive age. Research published in the Journal of Clinical Endocrinology and Metabolism estimates that thyroid disorders affect up to 4% of women trying to conceive, and subclinical thyroid dysfunction is even more common. Yet it's frequently underdiagnosed or undertreated in the context of fertility.
Many women trying to conceive have thyroid levels that are technically "normal" by standard lab ranges but not optimal for pregnancy.
If you've been struggling to conceive and haven't had your thyroid thoroughly evaluated, this could be the missing piece.
How Does Thyroid Affect Fertility?
Your thyroid produces hormones that regulate metabolism throughout your body. Every cell depends on thyroid hormones to function properly, including the cells of your reproductive system.
Thyroid hormones affect ovulation, the quality of your eggs, the health of your uterine lining, your body's ability to maintain early pregnancy, and fetal brain development.
When thyroid function is off, even slightly, any of these processes can be affected.
Ovulation
Your thyroid interacts closely with your reproductive hormones. Hypothyroidism (underactive thyroid) can disrupt ovulation, causing irregular or absent periods. Research in Human Reproduction found that even subclinical hypothyroidism, where TSH is elevated but not flagged as abnormal, can affect cycle regularity and ovulation.
Egg Quality
Thyroid hormones affect cellular energy production throughout the body, including in developing eggs. Research in Fertility and Sterility suggests that thyroid dysfunction may contribute to poor egg quality through its effects on mitochondrial function, though this connection requires more study.
Implantation and Early Pregnancy
This is where thyroid function may matter most. Adequate thyroid hormone is essential for implantation and early pregnancy maintenance. The first trimester is particularly vulnerable because the embryo depends entirely on maternal thyroid hormone before its own thyroid develops around 12 weeks.
Research published in the Journal of Clinical Endocrinology and Metabolism found that hypothyroidism is associated with increased rates of miscarriage, particularly in the first trimester. A meta-analysis in Thyroid showed that even subclinical hypothyroidism (TSH above 2.5) increases miscarriage risk by 60-80%, and that treating it reduces that risk.
Pregnancy Complications
Beyond early pregnancy, untreated thyroid dysfunction is associated with preeclampsia, gestational hypertension, preterm birth, and placental abruption. A study in Obstetrics and Gynecology found that women with untreated hypothyroidism had twice the rate of pregnancy complications compared to those with treated or normal thyroid function. Proper thyroid function throughout pregnancy matters for both maternal and fetal health.
What TSH Level Is Best for Getting Pregnant?
Here's where things get complicated. Standard lab reference ranges for thyroid function are based on the general population, not on what's optimal for fertility and pregnancy.
A TSH level of 4.0 might be considered "normal" by standard ranges but may be too high for optimal fertility. The American Thyroid Association recommends TSH under 2.5 when trying to conceive, and many reproductive endocrinologists prefer it under 2.0.
Research in the Journal of Clinical Endocrinology and Metabolism found that women with TSH above 2.5 had significantly lower pregnancy rates and higher miscarriage rates compared to women with TSH below 2.5, even though both groups fell within "normal" lab ranges.
This means you can have thyroid levels flagged as "normal" on your lab report while still having suboptimal thyroid function for pregnancy. If your doctor only glances at the "normal/abnormal" flag without looking at the actual numbers in context, a treatable issue might be missed.
What Thyroid Tests Should I Get for Fertility?
A complete thyroid assessment for fertility includes several tests.
TSH (Thyroid Stimulating Hormone): This is the most common screening test. It reflects how hard your pituitary is working to stimulate your thyroid. Higher TSH suggests your thyroid is underperforming. For fertility, optimal is generally under 2.5, with many practitioners preferring under 2.0.
Free T4: This is the main hormone your thyroid produces. It should be in the mid-to-upper part of the normal range.
Free T3: T3 is the active form of thyroid hormone. Some women have trouble converting T4 to T3, so checking both is important.
Thyroid Antibodies (TPO and TG): These test for autoimmune thyroid disease, most commonly Hashimoto's thyroiditis. You can have elevated antibodies even when TSH is normal, and this still affects fertility and pregnancy outcomes.
Many basic fertility workups only include TSH. If your TSH is "normal" but you have symptoms of thyroid dysfunction or unexplained fertility issues, ask for the full panel.
What Are Signs of Thyroid Problems?
Hypothyroidism symptoms are easy to dismiss or attribute to other causes. They include fatigue that sleep doesn't fix, feeling cold when others are comfortable, constipation, dry skin and hair, hair loss, weight gain or difficulty losing weight, brain fog and memory issues, depression, irregular or heavy periods, and low libido.
Hyperthyroidism (overactive thyroid) is less common but also affects fertility. Symptoms include anxiety and nervousness, rapid heartbeat, weight loss despite good appetite, feeling hot, tremors, and lighter or less frequent periods.
Many women with subclinical thyroid dysfunction have vague symptoms they've normalized. They've adapted to feeling tired, to gaining weight, to brain fog. They think it's just stress or aging. It's worth investigating whether thyroid function is a factor.
Can Hashimoto's Affect Fertility?
Hashimoto's thyroiditis is an autoimmune condition where your immune system attacks your thyroid. It's the most common cause of hypothyroidism and is more prevalent in women.
Hashimoto's can affect fertility even when TSH is normal. Research in Human Reproduction found that the autoimmune component itself, independent of thyroid hormone levels, is associated with increased miscarriage risk and reduced IVF success rates. The inflammation and immune dysregulation of Hashimoto's appear to directly impact reproductive function.
A study in the Journal of Clinical Endocrinology and Metabolism found that women with thyroid antibodies had a 3-4 times higher rate of miscarriage compared to women without antibodies, even when TSH was in normal range.
If you have elevated thyroid antibodies, even with normal TSH, this is worth addressing. Some practitioners recommend thyroid medication even with normal TSH when antibodies are elevated and you're trying to conceive. Others focus on reducing inflammation and supporting immune function.
What Is the Treatment for Thyroid and Infertility?
Conventional treatment for hypothyroidism is thyroid hormone replacement, typically levothyroxine (Synthroid). This is often very effective and well-tolerated. If you need it, taking thyroid medication is safe before and during pregnancy. In fact, it's essential for supporting a healthy pregnancy.
But medication isn't the only consideration. We also address factors that affect thyroid function.
Inflammation: Chronic inflammation worsens autoimmune thyroid disease and can interfere with thyroid hormone function. Reducing inflammation through diet, digestive health, and stress reduction supports thyroid function.
Digestive Health: A significant portion of T4 to T3 conversion happens in the digestive system. Poor digestive health can impair thyroid function even when the thyroid itself is working normally.
Nutrient Status: Your thyroid needs specific nutrients to function: iodine, selenium, zinc, iron, vitamin D, and B vitamins. Deficiencies in any of these can affect thyroid function.
Stress: Chronic stress affects the thyroid through multiple pathways. It can suppress TSH, impair T4 to T3 conversion, and worsen autoimmune activity.
Blood Sugar: Blood sugar instability stresses the adrenals, which in turn affects thyroid function. Stabilizing blood sugar often helps thyroid function improve.
Does Acupuncture Help Thyroid Function?
Acupuncture supports thyroid function through several mechanisms.
Research in the Journal of Alternative and Complementary Medicine found that acupuncture helps regulate the hypothalamic-pituitary-thyroid axis, the communication system between your brain and thyroid. It reduces inflammation, which is particularly important in autoimmune thyroid conditions. It supports the stress response and adrenal function, which interact closely with thyroid function. And it improves overall hormonal balance.
For women with Hashimoto's or subclinical hypothyroidism who are trying to conceive, we often recommend weekly acupuncture alongside any medical treatment. The goal is to optimize thyroid function and reduce the autoimmune inflammation that can affect fertility.
What This Looks Like in Practice
A 36-year-old patient came to us after a year of trying to conceive with one early miscarriage. Her basic fertility workup had been "normal." Her TSH was 3.8, which her doctor said was fine.
When we reviewed her history, several things stood out. She was tired all the time, even after sleeping eight hours. She was constipated. She'd gained weight gradually over the past few years despite not changing her diet. Her periods had gotten heavier. She felt cold constantly.
We recommended she request a full thyroid panel. Her results showed TSH of 3.8 (too high for optimal fertility), Free T4 at the low end of normal, and elevated TPO antibodies indicating Hashimoto's.
Her doctor agreed to start a low dose of levothyroxine. We supported her with weekly acupuncture and worked on reducing inflammation through diet and digestive support. We addressed her stress, which had been significant.
Over three months, her TSH came down to 1.8. Her energy improved dramatically. The constipation resolved. She felt like a different person.
She conceived four months after starting treatment. Her pregnancy was uncomplicated, and she delivered a healthy baby at 39 weeks.
Was it the thyroid medication? The acupuncture? The lifestyle changes? Probably all of it together. But addressing her thyroid, which had been dismissed as "normal," was the key that unlocked everything else.
Your Next Step
If you've been trying to conceive without success, or if you've experienced miscarriage, get your thyroid thoroughly evaluated. Don't accept "normal" without seeing the actual numbers. Ask for the full panel, not just TSH.
Optimal thyroid function for fertility means TSH under 2.5, ideally under 2.0, with Free T4 and Free T3 in the mid-to-upper normal range, and no elevated antibodies. If your numbers don't meet these targets, treatment may help.
And if you have symptoms of thyroid dysfunction, trust your body. Fatigue, weight gain, cold intolerance, and brain fog aren't just "getting older." They're signs worth investigating.
If you're concerned about your thyroid and fertility, we can help you understand your numbers and create a plan to optimize thyroid function. We work alongside your doctor, providing complementary support that addresses the factors affecting your thyroid health.
Learn more about our Fertility & Health path or contact us at 212.432.1110 or info@fafwellness.com.