What You May Not Know about Thyroid Health & Pregnancy?

By Chloe Dymond Young - BANT Registered Nutritional Therapist, IBCLC and ex-Midwife (17 years).

Thyroid Health and Fertility: What Every Woman Trying to Conceive Should Know:

Your thyroid - a small, butterfly-shaped gland in your neck plays a big role in your fertility and reproductive health. It regulates your metabolism, menstrual cycle, ovulation, implantation, and pregnancy maintenance.

Even subtle imbalances in thyroid hormones can impact your chances of conception, increase miscarriage risk, or affect the health of your baby. Yet thyroid issues are often overlooked, especially if levels fall within the “normal” lab reference range.

In this post, we’ll explore the connection between thyroid health and fertility, including subclinical hypothyroidism, thyroid antibodies, nutritional support, genetics, and what you can do to optimise your thyroid before pregnancy.

Why Thyroid Hormones Matter for Fertility:

Thyroid hormones (primarily T3 and T4) influence:

  • Ovulation: Low thyroid hormone levels can suppress ovulation or lead to irregular cycles

  • Luteal phase length: A short luteal phase can reduce implantation success

  • Progesterone production: TSH imbalances may disrupt corpus luteum function

  • Egg quality: Mitochondria in the oocyte are sensitive to metabolic and thyroid-related stress

  • Endometrial receptivity: Adequate T3 is needed for a receptive uterine lining

A 2017 meta-analysis in Human Reproduction Update found that even mild hypothyroidism is associated with increased miscarriage rates and reduced live birth rates, especially during assisted reproduction.

What Is Subclinical Hypothyroidism?

Subclinical hypothyroidism is a mild form of under-active thyroid where:

  • TSH (thyroid-stimulating hormone) is elevated (commonly 2.5–4.5 mIU/L)

  • Free T4 and Free T3 are within the “normal” range

  • Symptoms may be subtle or absent

Subclinical levels can still affect fertility and early pregnancy, especially if thyroid antibodies are present.

Functional medicine and fertility guidelines often recommend keeping TSH below 2.5 mIU/L when trying to conceive or in early pregnancy - even though many labs consider up to 4.5 as normal.

Thyroid Autoimmunity and Antibodies:

Even with normal thyroid hormone levels, the presence of thyroid antibodies can increase the risk of:

  • Infertility and implantation failure

  • Recurrent miscarriage

  • Thyroid dysfunction in pregnancy or postpartum

Common antibodies tested:

  • TPO (Thyroid Peroxidase Antibodies)

  • TG (Thyroglobulin Antibodies)

Autoimmune thyroid conditions like Hashimoto’s thyroiditis are often underdiagnosed, particularly in women with family history or unexplained fertility issues.

A 2011 study in The Journal of Clinical Endocrinology & Metabolism found that euthyroid women (normal thyroid levels) with thyroid antibodies had double the miscarriage rate compared to antibody-negative women.

Genetic SNPs & Family History:

Genetics can significantly impact thyroid function. You're at higher risk of thyroid imbalances if you have:

  • A family history of thyroid disease (especially in mothers or sisters)

  • Genetic SNPs such as:

    • DIO1/DIO2: affect conversion of T4 to active T3

    • MTHFR: reduces methylation needed for detoxifying thyroid-disrupting toxins

    • TPO gene variants: increase risk of autoimmunity

    • FUT2: affects B12 and gut microbiome health, indirectly impacting thyroid health

If you have these SNPs or family history, preconception screening and nutritional support are essential.

Nutrition to Support Thyroid Health:

  1. Selenium
    • Helps reduce thyroid antibodies and supports hormone conversion
    Sources: Brazil nuts, eggs, sunflower seeds

  2. Iodine
    • Needed to produce thyroid hormones T3 and T4 (be cautious as too much iodine in the presence of Hashimoto’s thyroiditis can increase TPO anti-bodies and worsen inflammation).
    Sources: Seaweed (kelp, nori), fish, milk

  3. Zinc
    • Essential for T3 hormone receptor function
    Sources: Pumpkin seeds, seafood, lentils

  4. Iron
    • Important for thyroid hormone synthesis
    Sources: Red meat, lentils, spinach (pair non-heme iron with vitamin C for better absorption)

  5. Vitamin D
    • Modulates the immune system and supports thyroid health
    Sources: Sunshine, fatty fish, vitamin D3 supplements

  6. Vitamin B12
    • Supports methylation and energy production
    Sources: Eggs, Seafood, B12 supplements (especially important for vegans)

  7. Tyrosine
    • Amino acid needed to produce T3 and T4
    Sources: Poultry, eggs, almonds, sesame seeds

    A 2020 review in Nutrients highlighted selenium’s role in lowering TPO antibodies and improving thyroid function in women of reproductive age.

Lifestyle and Gut Health Factors:

  • Inflammation and leaky gut can trigger autoimmune thyroid issues

  • Gut dysbiosis reduces nutrient absorption (especially B12, iron, zinc)

  • Gluten sensitivity (especially in Hashimoto’s) can trigger immune flare-ups

  • Chronic stress raises cortisol, which suppresses thyroid function

Lifestyle Tips to Support Thyroid and Fertility:

  • Sleep: 7–9 hours to support hormone regulation

  • Manage stress: Use breath-work, meditation, yoga, or walking

  • Avoid endocrine disruptors: Limit plastics, pesticides, BPA, and synthetic fragrances

  • Movement: Moderate exercise boosts thyroid function, but avoid overtraining

Functional Thyroid Testing Checklist

Ask your practitioner for a comprehensive thyroid panel, including:

  • TSH

  • Free T3

  • Free T4

  • Reverse T3 (if fatigue is present)

  • TPO & TG antibodies

  • Iron, folate, zinc, B12 and vitamin D levels

Finally:

Your thyroid health is central to your fertility and pregnancy journey. Even subtle shifts in hormones or the presence of antibodies can create barriers to conception, but with targeted testing, personalised nutrition, and lifestyle support, balance can often be restored.

If you need help navigating thyroid help, book into my Women & Baby clinic via my services page or for a more comprehensive programme, BOOK A FREE DISCOVERY CALL to find out how I can help you.

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