Thyroid Health &Fertility: Why Thyroid Function Matters Before and During Pregnancy

If conceiving and pregnancy are in your future plans, there’s one system in your body that determines far more than most women realize, and is often overlooked in conventional care – your thyroid. This small gland in your neck governs metabolism, hormone signaling, and cellular energy. In the context of fertility and pregnancy, its role is even more critical – it influences ovulation, ease in conceiving, and maintaining a pregnancy. And yet, thyroid dysfunction is one of the most underdiagnosed and undertreated drivers of fertility challenges and pregnancy complications.

The thyroid and reproductive systems are interconnected and the consequences of poor, or even suboptimal (i.e. “not-so-great-but-still-within-lab-reference-ranges”), thyroid function can include irregular or absent ovulation, luteal phase defects (i.e. inadequate development of uterine lining), elevated prolactin (which suppresses ovulation) and altered estrogen metabolism. Even in cases labeled as “unexplained infertility”, or “subfertility”, underlying thyroid dysfunction is often a contributing factor. Research shows that both overt and subclinical Hypothyroidism is associated with reduced fertility and impaired ovarian function (1).

Why “Normal” Thyroid Labs May Not Be Enough

Many women are told their thyroid is “normal”- but standard lab ranges are not designed for fertility optimization.

For preconception and early pregnancy:

•  TSH is often ideally less than 2.5 mIU/L
•  Free T4 and Free T3 should be optimal, not just within range
•  Thyroid antibodies should always be assessed

Autoimmune thyroid conditions like Hashimoto’s thyroiditis are associated with increased miscarriage risk, implantation challenges, poorer fertility outcomes- even when TSH appears normal (2).

The Critical Role of Thyroid Hormones in Early Pregnancy

In early pregnancy, the baby depends entirely on the mother’s thyroid hormones. Maternal T4 is essential for the baby’s brain development, nervous system formation, cellular growth and differentiation. Low maternal thyroid hormone levels during this window have been associated with impaired cognitive outcomes and changes in brain development in offspring (3).

What Happens When Thyroid Function Isn’t Optimized?

Suboptimal thyroid function has been linked to miscarriage and recurrent pregnancy loss (i.e.“subfertility”), preterm birth, gestational hypertension and preeclampsia, low birth weight and placental complications. Women with elevated TSH early in pregnancy (i.e. lower thyroid function) have a significantly increased risk of miscarriage, particularly above 2.5 mIU/L (4). Even subclinical hypothyroidism is associated with higher rates of adverse pregnancy outcomes (5).

Why This Matters Before You Get Pregnant

Pregnancy increases thyroid hormone demands by up to 50% (6). If your thyroid is already under strain then conception may be more difficult, early pregnancy may not be adequately supported and you hormonal adaptation may be impaired. This is why optimizing thyroid function before conception- not after- is critical.

A Functional, Root-Cause Approach to Thyroid Optimization

1. Nutrient Optimization

Key nutrients for thyroid function include:

•  Iodine
•  Selenium
•  Iron
•  Zinc

2. Autoimmune Modulation

In cases involving Hashimoto’s thyroiditis:

•  Identify and reduce immune triggers
•  Support gut health
•  Lower systemic inflammation

3. Liver & Gut Health

Thyroid hormone conversion (T4 → T3) depends on:

•  Liver function
•  Gut microbiome balance
•  Key nutrient levels

4. Environmental Medicine

Reducing exposure to endocrine disruptors (plastics, heavy metals, pesticides) is especially important during preconception.

What Testing Should Be Considered?

A comprehensive thyroid panel may include:

•  TSH
•  Free T4
•  Free T3
•  Thyroid antibodies (TPO, Tg)
•  Iron studies (ferritin)

This allows for a precision-based, individualized fertility strategy.

The Bottom Line

Your thyroid is not just another lab marker—it is a core regulator of fertility, pregnancy health, and fetal development. Optimizing thyroid function can improve chances of conception, reduce miscarriage risk, support a healthier and more comfortable pregnancy as well as enhance long-term outcomes for your child.

If you’re preparing for pregnancy or navigating fertility challenges, a personalized assessment and a preconception plan can make a measurable difference.

References

1. Alexander EK, Pearce EN, Brent GA, et al. 2017 guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315-389. doi:10.1089/thy.2016.0457

2. Thangaratinam S, Tan A, Knox E, Kilby MD, Franklyn J, Coomarasamy A. Association between thyroid autoantibodies and miscarriage and preterm birth: meta-analysis of evidence. BMJ. 2011;342:d2616. doi:10.1136/bmj.d2616

3. Korevaar TIM, Muetzel R, Medici M, et al. Association of maternal thyroid function during early pregnancy with offspring IQ and brain morphology. Lancet Diabetes Endocrinol. 2017;5(1):35-43. doi:10.1016/S2213-8587(16)30327-7

4. Negro R, Schwartz A, Gismondi R, Tinelli A, Mangieri T, Stagnaro-Green A. Universal screening versus case finding for detection and treatment of thyroid hormonal dysfunction during pregnancy. J Clin Endocrinol Metab. 2010;95(4):1699-1707. doi:10.1210/jc.2009-2009

5. Maraka S, Ospina NMS, O’Keeffe DT, et al. Subclinical hypothyroidism in pregnancy: a systematic review and meta-analysis. Thyroid. 2016;26(4):580-590. doi:10.1089/thy.2015.0418

6. Glinoer D. The regulation of thyroid function during normal pregnancy: importance of the iodine nutrition status. Best Pract Res Clin Endocrinol