Thyroid disorders and pregnancy
Thyroid Hormones and Their Role in Reproductive Health
In the Western world, hypothyroidism—a deficiency in thyroid hormone—is often caused by autoimmune thyroid disease, such as Hashimoto's thyroiditis. This condition involves the production of thyroid-specific antibodies, including anti-TPO (against thyroid peroxidase, an enzyme essential for hormone synthesis) and anti-Tg (against thyroglobulin, the protein that stores thyroid hormones). These antibodies trigger an immune reaction that damages the thyroid gland, leading to reduced hormone production.
Beyond thyroid function, these antibodies may impact overall reproductive health, including fertility and pregnancy outcomes, as part of a broader autoimmune process.
Diagnosing Hypothyroidism
Hypothyroidism is typically diagnosed by measuring TSH (thyroid-stimulating hormone) levels. A TSH level above the reference range indicates that the pituitary gland is overcompensating for a poorly functioning thyroid.
Currently there is no routine screening for thyroid disorders for women by the NHS as part of pre-conception or antenatal care, but it is part of investigation for subfertility and recurrent pregnancy loss. At Sirona Health we include advanced thyroid testing into our preconception panel so that all women can be screened when they are considering a pregnancy. This will help them to determine if they should consider repeat thyroid testing during pregnancy, or whether they would benefit from treatment with levothyroxine.
For preconception and early pregnancy, the recommended upper TSH limit has shifted from 4.0–5.0 mIU/L to a more conservative 2.5 mIU/L for high-risk individuals, such as those with subfertility or recurrent pregnancy loss (RPL). However, defining a universal TSH threshold remains complex due to factors like iodine intake, ethnicity, and body mass index (BMI).
The Royal College of Obstetricians and Gynaecologists (RCOG) categorises TSH levels as follows:
Upper normal: TSH 2.5-4.0 mlU/L
Mild–moderate subclinical hypothyroidism: TSH >4.0–10.0 mIU/L
Overt hypothyroidism: TSH >10.0 mIU/L (even if free T4 is normal)
If the TSH level is elevated, a repeat test is recommended 4–6 weeks later to confirm the diagnosis. This helps rule out temporary thyroid dysfunction due to factors like illness, stress, or medications.
Thyroid Antibodies in Reproductive-Age Women
Thyroid peroxidase antibodies (anti-TPO) are present in 5%–20% of women of childbearing age. Prevalence increases in women with subfertility (10%–31%) and recurrent pregnancy loss (17%–33%) compared to the general population.
Subclinical Hypothyroidism and Fertility
While subclinical hypothyroidism (SCH) is linked to slightly higher TSH levels in women with subfertility, there is no definitive evidence that mild elevations directly cause infertility. Current guidelines recommend investigating and managing thyroid function as part of a broader assessment of fertility.
Subclinical Hypothyroidism and Recurrent Pregnancy Loss
Overt hypothyroidism is a well-documented risk factor for miscarriage. TSH levels in the mild–moderate range (>4.0–10.0 mIU/L) during pregnancy are also associated with an increased miscarriage risk. The presence of anti-TPO antibodies amplifies this risk, and therefore for women planning pregnancy, treating mild–moderate SCH with levothyroxine is recommended. Similarly, those with TSH in the upper normal range (2.5–4.0 mIU/L) and positive anti-TPO antibodies may benefit from levothyroxine therapy but there is disagreement internationally about how to manage these women so an individualised discussion is important.
Treatment Recommendations
TSH >4.0–10.0 mIU/L (particularly if anti-TPO positivity): Start levothyroxine at 1.0–1.2 μg/kg body weight per day.
TSH 2.5–4.0 mIU/L with anti-TPO positivity: Consider starting levothyroxine.
Managing Subclinical Hypothyroidism During Pregnancy
For women already on levothyroxine preconception, the thyroid hormone demand increases by 20%–30% during pregnancy. Upon confirming pregnancy:
Increase dose: Double the usual dose on two days a week (e.g., Saturday and Sunday).
Alternatively, increase the daily dose by:
25 μg for those taking ≤100 μg daily.
50 μg for those taking >100 μg daily.
Monitoring:
Measure TSH at 7–9 weeks gestation and continue monitoring every 4–6 weeks until 34 weeks.
For women with anti-TPO positivity but normal TSH levels, monitor TSH closely throughout pregnancy.
Conclusion
Subclinical hypothyroidism and thyroid autoimmunity significantly influence reproductive health, particularly in women with subfertility or recurrent pregnancy loss. Early diagnosis, individualised treatment, and regular monitoring are essential to optimise outcomes for both mother and baby.
If you're navigating fertility challenges or planning pregnancy, consult Sirona Health to assess your thyroid health and develop a tailored management plan.