Thyroid Function and Fertility: Why TSH Matters More Than Your Doctor May Have Told You

By Dr (TCM) Attilio D'Alberto | Traditional Chinese Medicine Practitioner, Wokingham

Thyroid function is one of the most consistently overlooked contributors to fertility difficulties, and one of the most important to address. The thyroid gland produces hormones that regulate metabolism throughout the body — including the reproductive system — and even subtle disruptions to thyroid function can impair ovulation, reduce egg quality, prevent implantation, and increase the risk of miscarriage. Yet millions of women with suboptimal thyroid function receive normal results on standard testing, because conventional laboratory reference ranges are far too wide for reproductive purposes.

Understanding how the thyroid affects fertility, what the optimal ranges are for conception, and how traditional Chinese medicine (TCM) supports thyroid function is essential for any woman who has unexplained infertility, irregular cycles, recurrent miscarriage, or who simply wants to optimise her chances of conceiving.

On this page

1. Thyroid hormones and their roles
2. TSH and fertility: what the research says
3. Hypothyroidism and fertility
4. Hyperthyroidism and fertility
5. Hashimoto's thyroiditis
6. What thyroid testing should include
7. TCM understanding of the thyroid
8. Acupuncture for thyroid disorders
9. Chinese herbal medicine
10. Diet and key nutrients
11. Thyroid function in pregnancy
12. My Fertility Guide
13. References

1. Thyroid hormones and their roles

The thyroid gland produces two primary hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the predominant form produced by the gland, but it is relatively inactive — it is converted to the active form T3 in the peripheral tissues, primarily in the liver. T3 is the hormone that enters cells and drives metabolic activity at the cellular level.

The pituitary gland controls thyroid output by releasing thyroid stimulating hormone (TSH). When thyroid hormone levels are adequate, TSH is suppressed. When they fall — or when the body perceives a need for more — TSH rises to stimulate the thyroid to produce more T4 and T3. TSH is therefore an inverse marker: high TSH means the thyroid is underperforming (or being signalled to work harder); low TSH means the thyroid is overproducing (or being adequately supplemented).

Thyroid hormones influence virtually every system in the body. Of particular relevance to fertility:

• T3 directly promotes FSH receptor sensitivity in the ovaries, supporting follicle development and egg maturation
• Thyroid hormones regulate progesterone production — T3 increases the number of LH receptors on the corpus luteum, which is required for adequate luteal phase progesterone output
• Thyroid hormones are essential for endometrial development and receptivity
• Adequate thyroid function is required for normal implantation and early embryo development
• T3 is critical for normal fetal brain development in the first trimester, before the fetal thyroid becomes functional

2. TSH and fertility: what the research says

Conventional laboratory reference ranges for TSH are typically 0.4–4.5 mIU/L or similar, depending on the laboratory. A result within this range is reported as normal and no treatment is offered. However, fertility research consistently shows that TSH above 2.5 mIU/L is associated with poorer reproductive outcomes.

A landmark study published in Fertility and Sterility found that women with TSH between 2.5 and 5.0 mIU/L had significantly higher rates of miscarriage compared to those with TSH below 2.5 mIU/L. Multiple subsequent studies have confirmed that TSH above 2.5 mIU/L is associated with:

• Reduced implantation rates in IVF
• Higher rates of early pregnancy loss
• Reduced clinical pregnancy rates per cycle
• Longer time to conception in natural conception cohorts

The American Thyroid Association guidelines for thyroid management in pregnancy now recommend that TSH should be below 2.5 mIU/L in the first trimester and below 3.0 mIU/L in the second and third trimesters. Many fertility specialists apply the 2.5 mIU/L threshold from the preconception period. If your TSH is between 2.5 and 4.5 mIU/L and you have been told your thyroid is normal, it is worth discussing this with your GP or fertility specialist specifically in the context of conception.

3. Hypothyroidism and fertility

Hypothyroidism — an underactive thyroid — is the more common thyroid condition in reproductive-age women. Both overt hypothyroidism (TSH above 4.5 mIU/L with low T4) and subclinical hypothyroidism (TSH elevated but T4 normal) are associated with fertility impairment.

The mechanisms through which hypothyroidism impairs fertility include:

• Disrupted ovulation: Low thyroid hormones impair the mid-cycle LH surge required for ovulation, and may cause anovulatory cycles without obvious external signs of cycle disruption.
• Shortened luteal phase: Reduced LH receptor expression on the corpus luteum (driven by low T3) decreases progesterone production, creating a functionally short or inadequate luteal phase that does not support implantation.
• Elevated prolactin: TRH (thyrotropin-releasing hormone), which rises when the thyroid is underactive, also stimulates prolactin release from the pituitary. Elevated prolactin suppresses GnRH and LH, further disrupting ovulation.
• Poor endometrial development: Thyroid hormones are required for normal endometrial response to oestrogen and progesterone. Deficiency results in a less receptive uterine lining.
• Increased miscarriage risk: Even mild hypothyroidism significantly increases the risk of early pregnancy loss, both through direct hormonal effects on embryo development and through associated autoimmune mechanisms.

Symptoms of hypothyroidism that are commonly ignored or attributed to other causes in fertility patients include fatigue, weight gain despite normal diet, cold intolerance, constipation, hair thinning, dry skin, slow heart rate, low mood, and heavy or irregular periods. Many women with mild hypothyroidism have only a few of these symptoms at low severity.

4. Hyperthyroidism and fertility

Hyperthyroidism — an overactive thyroid, most commonly due to Graves' disease — also impairs fertility, though it is less common in this context than hypothyroidism. Elevated thyroid hormones disrupt the normal pituitary release of LH and FSH, cause cycle irregularity, and are associated with miscarriage and preterm birth. Women with known Graves' disease who are planning pregnancy should ensure their hyperthyroidism is well controlled before conceiving, as both the condition and its treatment can affect pregnancy outcomes.

5. Hashimoto's thyroiditis

Hashimoto's thyroiditis — autoimmune thyroid disease — is the most common cause of hypothyroidism in developed countries. In Hashimoto's, the immune system produces antibodies (anti-TPO and anti-thyroglobulin) that gradually destroy the thyroid gland, leading to progressive reduction in thyroid hormone production over months or years.

Thyroid antibodies have an independent association with fertility difficulties and pregnancy loss even when TSH and T4 are normal. Women with positive thyroid antibodies have approximately twice the miscarriage rate of antibody-negative women, even when thyroid function appears preserved. The precise mechanism is not fully established but may involve generalised immune dysregulation that also affects implantation and the embryo.

Standard testing often does not include thyroid antibodies unless TSH is abnormal. Requesting anti-TPO and anti-thyroglobulin antibody testing is worth considering for any woman with unexplained infertility, recurrent miscarriage, or a personal or family history of autoimmune disease.

6. What thyroid testing should include

A comprehensive thyroid panel for a woman trying to conceive should include:

• TSH: Optimal target <2.5 mIU/L for conception; <2.0 mIU/L is preferable
• Free T4: Should be in the upper half of the reference range
• Free T3: Tells you whether conversion of T4 to the active T3 form is adequate
• Anti-TPO antibodies: Elevated in Hashimoto's; associated with miscarriage risk independently of TSH
• Anti-thyroglobulin antibodies: Second marker for Hashimoto's
• Reverse T3 (rT3): When chronic stress or illness is present, T4 may be converted to inactive rT3 rather than active T3 — an important pattern to identify when free T3 is low despite normal T4

Note that standard NHS thyroid testing typically includes only TSH and sometimes T4. If you are seeing a fertility specialist or GP about conception difficulties, you may need to specifically request free T3 and thyroid antibodies.

7. TCM understanding of the thyroid

Traditional Chinese medicine does not have an equivalent anatomical organ to the thyroid, but the functions of thyroid hormones correspond closely to the TCM concept of Kidney Yang — the warming, activating, metabolic force that drives all physiological processes in the body. Hypothyroidism maps closely onto a Kidney Yang deficiency pattern: coldness, fatigue, weight gain, slow metabolism, low mood, and reproductive difficulty.

The Spleen's role in transformation and transportation is also relevant — poor thyroid function impairs the Spleen's ability to digest and transform food into Blood and energy, leading to the accumulation of phlegm-damp (which in physical terms corresponds to the fluid retention, sluggish lymphatics, and myxoedema of hypothyroidism).

Autoimmune thyroid disease (Hashimoto's) in TCM is associated with patterns of Yin deficiency generating empty Heat, combined with Liver Qi stagnation — the immune dysregulation reflects an internal imbalance in which the body's regulatory mechanisms have been overwhelmed by chronic stress, constitutional deficiency, or environmental factors.

8. Acupuncture for thyroid disorders

Acupuncture supports thyroid function through several pathways:

HPA axis modulation: Chronic stress is a major driver of thyroid dysfunction. Elevated cortisol reduces T3 production, increases reverse T3 conversion, and suppresses thyroid hormone receptor sensitivity. Acupuncture normalises the stress response and lowers cortisol, directly improving the thyroid hormonal environment.

Immune regulation: In Hashimoto's thyroiditis, acupuncture has been shown in several studies to reduce circulating thyroid antibody levels (anti-TPO), suggesting a genuine immunomodulatory effect. This is consistent with acupuncture's established effects on inflammatory cytokines and T-regulatory cell activity.

Improvement of peripheral T4-to-T3 conversion: Acupuncture stimulates the liver and kidney meridians, which correspond to the organs primarily responsible for T4 conversion in TCM terms — and indeed in physiological terms (the liver is the primary site of T4 to T3 conversion). Supporting these organ systems promotes more efficient activation of thyroid hormone.

Direct regulation of TSH: Some studies have shown that acupuncture can lower elevated TSH in subclinical hypothyroid patients, likely through hypothalamic modulation of TRH and subsequent pituitary TSH secretion.

9. Chinese herbal medicine

Chinese herbal prescriptions for thyroid-related fertility problems are directed at the underlying TCM pattern:

• You Gui Wan (Restore the Right Pill): Warms Kidney Yang, tonifies Qi and Blood — the primary formula for hypothyroid-type presentations with cold, fatigue, and reproductive impairment.
• Jin Gui Shen Qi Wan (Kidney Qi Pill from the Golden Cabinet): Addresses the root Kidney Yang deficiency with a gentler warming action — suitable where cold signs are present but less severe, or where both Yin and Yang are deficient.
• Er Xian Tang (Two Immortals Decoction): Balances Kidney Yin and Yang while clearing empty Heat — appropriate in Hashimoto's or mixed hypothyroid/autoimmune presentations.
• Xiao Yao San (Free and Easy Wanderer) modifications: Addresses the Liver Qi stagnation component common in autoimmune thyroid disease and chronic stress-driven thyroid disruption.
• Si Jun Zi Tang (Four Gentlemen): Strengthens Spleen Qi and supports the transformation and transportation functions impaired by poor thyroid activity.

Individual herbs of relevance include Fu Zi (Radix Aconiti Praeparata) for strong Yang warming, Wu Wei Zi (Schisandra) for adrenal and thyroid support, and Huang Qi (Astragalus) for immune modulation and Qi tonification.

10. Diet and key nutrients

Several nutrients are essential for thyroid hormone synthesis and conversion:

• Iodine: The building block of thyroid hormones. Deficiency is widespread in the UK (dairy restriction and reduced seafood intake are common contributing factors). Adequate iodine intake from seaweed, dairy, eggs, and seafood — or supplementation of 150mcg daily (the recommended adult intake) — is important. Note: excessive iodine can worsen autoimmune thyroid disease; do not take high-dose iodine supplements without testing first.
• Selenium: Required for the conversion of T4 to T3 and for the production of glutathione peroxidase, which protects the thyroid from oxidative damage. Studies have shown that selenium supplementation (200mcg daily) reduces anti-TPO antibody levels in Hashimoto's and improves thyroid function. Brazil nuts provide approximately 50–70mcg selenium each.
• Zinc: Supports both thyroid hormone synthesis and conversion. Common in meats, shellfish (particularly oysters), nuts and seeds.
• Iron: Iron deficiency impairs thyroid peroxidase (the enzyme that synthesises thyroid hormones). Women with heavy periods and Hashimoto's are at particular risk.
• Vitamin D: Deficiency is associated with autoimmune thyroid disease. Supplementation reduces antibody levels and supports immune regulation. See vitamin D and fertility.
• Avoid goitrogens in excess: Raw cruciferous vegetables (broccoli, kale, Brussels sprouts, cabbage) contain goitrogens that can impair thyroid iodine uptake in large amounts. Cooking neutralises most of the effect; moderate intake of cooked cruciferous vegetables is perfectly safe and beneficial.

11. Thyroid function in pregnancy

Thyroid demands increase significantly in pregnancy because hCG (the pregnancy hormone) stimulates the thyroid in early pregnancy, and the maternal thyroid must supply T3 and T4 to support fetal brain development before the fetal thyroid becomes active at around 10–12 weeks. Women with pre-existing hypothyroidism typically need a 30–50% increase in their levothyroxine dose from the point of conception. This is why it is important to have an agreed plan with your GP before becoming pregnant — dose adjustment from the very first positive test is often necessary.

Women on levothyroxine should also note that iron supplements (commonly prescribed in pregnancy) can impair levothyroxine absorption if taken simultaneously. Separate the doses by at least four hours.

Untreated or inadequately treated hypothyroidism in pregnancy is associated with miscarriage, preterm birth, placental abruption, gestational hypertension, and neurodevelopmental impairment in the child. Managing thyroid function optimally before and during pregnancy is one of the single most impactful things that can be done for both maternal and fetal outcomes.

12. My Fertility Guide

13. References

• Negro R, et al. Levothyroxine treatment in euthyroid pregnant women with autoimmune thyroid disease. J Clin Endocrinol Metab. 2006;91(7):2587–2591.
• Stagnaro-Green A, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2011;21(10):1081–1125.
• Thangaratinam S, et al. Association between thyroid autoantibodies and miscarriage and preterm birth. BMJ. 2011;342:d2616.
• Kolodziejczyk B, et al. The adrenal androgen DHEA and thyroid hormones: possible interactions and reproductive implications. Reprod Biol. 2004;4(2):145–156.
• Toulis KA, et al. Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010;20(10):1163–1173.