Your thyroid gland plays a surprisingly important role in fertility. Even mildly abnormal thyroid function can disrupt ovulation, increase miscarriage risk and affect early pregnancy. The good news: thyroid issues are usually easy to identify and treat.
How the Thyroid Influences Fertility
The thyroid is a small butterfly-shaped gland in your neck that produces hormones (T3 and T4) controlling metabolism in every cell of your body. Reproductive tissues are particularly sensitive to thyroid hormone. The thyroid interacts closely with the hypothalamic-pituitary-ovarian axis that governs ovulation. When thyroid hormone is too low or too high, it can disrupt the menstrual cycle, prevent ovulation, impair egg quality and increase miscarriage risk. Thyroid hormone is also essential for fetal brain development, particularly in the first trimester before the baby produces its own.
Common Thyroid Conditions Affecting Fertility
Hypothyroidism (underactive thyroid): The most common thyroid problem in women of reproductive age. Causes irregular or absent periods, anovulation, low libido and increased miscarriage risk.
Hashimoto's thyroiditis: An autoimmune condition where the body attacks the thyroid. Can cause hypothyroidism and is associated with higher miscarriage rates even when thyroid hormone levels appear normal.
Hyperthyroidism (overactive thyroid): Less common but also affects fertility. Causes irregular periods, weight loss and can increase pregnancy complications.
Subclinical hypothyroidism: Mildly elevated TSH with normal T4. Often missed on routine testing but can still affect fertility, especially with positive thyroid antibodies.
Thyroid antibodies (TPO, Tg): Can be present even when thyroid hormone levels are normal and are independently associated with miscarriage and reduced IVF success.
Who Should Have Thyroid Testing?
Anyone trying to conceive — TSH testing is now considered routine in fertility assessment
Women with irregular or absent periods
Anyone with a personal or family history of thyroid disease
Women with autoimmune conditions (lupus, type 1 diabetes, coeliac disease)
Women who have had a miscarriage or recurrent pregnancy loss
Patients about to start IVF — most clinics test routinely
Women experiencing symptoms: fatigue, weight changes, hair loss, cold intolerance, mood changes
Optimal Thyroid Levels for Fertility
The general population reference range for TSH is roughly 0.4-4.0 mIU/L, but for women trying to conceive or in early pregnancy, most fertility specialists aim for a TSH below 2.5 mIU/L. Some advocate for levels under 2.0. If TPO antibodies are positive, treatment thresholds may be lower. Treatment is usually a daily dose of levothyroxine, which is safe in pregnancy and can dramatically improve fertility outcomes. After starting or adjusting treatment, levels are usually rechecked after 6-8 weeks. Once pregnant, dose requirements typically increase by around 30%, so close monitoring continues.
Educational information, not medical advice. For advice about your own situation, talk to your doctor.