Thyroid health
How Hypothyroidism Can Affect Fertility and Pregnancy
How hypothyroidism affects fertility, pregnancy and the months after birth, why early testing matters and how to prepare your thyroid health.
Hypothyroidism, when the thyroid gland does not make enough thyroid hormone, is one of the most common endocrine disorders, particularly among women of childbearing age. A 2012 study published in the International Journal of Applied and Basic Medical Research (source) notes that about 2 to 4 percent of women in this group have low thyroid hormone levels, which means millions of women worldwide.
Why it matters
Thyroid dysfunction can disrupt ovulation, cause irregular periods, raise the risk of preeclampsia, miscarriage and preterm delivery, and affect the baby's brain development, so early diagnosis and treatment are important.
This article explains how hypothyroidism affects fertility and pregnancy at three stages (before conception, during pregnancy and after birth) and what you can do at each stage to protect your health and your baby's.
- Phase 1
Fertility before pregnancy
- Phase 2
Pregnancy management
- Phase 3
Postpartum thyroiditis
- 2–4%Women of childbearing age affected
- 3Life stages covered: before, during and after pregnancy
- 20%Risk of permanent hypothyroidism after postpartum thyroiditis
- 4wkBetween TSH checks until mid-pregnancy
Contents 10 sections
Before pregnancy: how hypothyroidism affects fertility
Hypothyroidism is often underestimated as a cause of difficulty conceiving. The thyroid gland plays a key role in regulating the menstrual cycle and ovulation. Low levels of thyroxine (T4) or raised thyrotropin-releasing hormone (TRH) may increase prolactin production, leading to anovulation (no egg release) or irregular ovulation.
A common complication is a shortened luteal phase (the second half of the menstrual cycle) which may not give a fertilised egg sufficient time to implant in the uterus.
Low basal body temperature
May make conditions for implantation less favourable and the fertile window harder to track, although the evidence for this is limited.
Raised TPO antibodies
Indicates autoimmune thyroiditis (Hashimoto's), which can impair ovarian function and significantly increase miscarriage risk.
Ovarian cysts
Thyroid dysfunction can promote ovarian cyst development, further contributing to infertility.
Why early diagnosis matters
Checking thyroid-stimulating hormone (TSH) and free T4 levels before pregnancy is important. Guidelines recommend reaching a euthyroid state (normal thyroid function) before conception to improve fertility and lower pregnancy risks.
Who should be screened?
Women planning pregnancy who have: a history of low thyroid hormones or pregnancy loss; family history of thyroid dysfunction or autoimmune diseases; irregular periods or difficulty conceiving for more than 6 months.
Key tests before conception
TSH level: the main test; if you already take levothyroxine, the dose is adjusted before conception so that TSH is between the lower reference limit and 2.5 mIU/L
Free T4 (fT4): checks that thyroid hormone levels are high enough for early embryo development
TPO antibodies: identify autoimmune thyroid disease, which increases miscarriage risk
How to optimise thyroid health before pregnancy
Steps before you try to conceive
- Regular testing
Have your TSH and T4 levels checked during the preconception period, ideally 3–6 months before attempting to conceive.
- Coordinated care
Ask your endocrinologist and obstetrician to work together on one coordinated thyroid management plan.
- Reach normal thyroid levels
Make sure your TSH is within the target range before conception. If your TSH is raised, even mildly, ask your doctor without delay whether you need treatment: this depends on the TSH level, your TPO antibody result and whether you are planning IVF.
- Correct nutrient deficiencies
Make sure you get enough iodine, selenium, zinc and vitamin D, which are important for thyroid function and fertility.
- Act early on symptoms
Have hypothyroid symptoms checked early to help pregnancy go smoothly and lower the risk of early pregnancy loss.
Looking after thyroid health before conception may improve fertility and lower the risk of early pregnancy complications.
Managing hypothyroidism during pregnancy
Pregnancy brings extra challenges for women with hypothyroidism. Many early pregnancy symptoms (such as extreme fatigue, weight gain, sensitivity to cold, muscle cramps and difficulty concentrating) overlap with common signs of hypothyroidism, so close monitoring is important.
Why thyroid hormones matter in pregnancy
During the first trimester, the baby relies entirely on the mother for thyroid hormones, because the baby's thyroid gland is not yet working. These hormones are essential for the baby's brain development and growth, so good control of the mother's thyroid is one of the most important factors in early fetal development.
With well-controlled hypothyroidism
- Normal fetal brain and nervous system development
- Reduced risk of pregnancy complications
- Better maternal energy and wellbeing
- Normal birth weight and gestational age
With uncontrolled hypothyroidism
- Impaired fetal neurodevelopment
- Increased risk of miscarriage and preterm birth
- Maternal anaemia and preeclampsia risk
- Low birth weight and developmental delays
Treatment changes during pregnancy
Managing hypothyroidism during pregnancy often involves adjustments to treatment plans to meet the body's increasing demands. Thyroid hormone requirements typically increase by 25–50% during pregnancy.
Frequent monitoring
TSH levels should be checked about every 4 weeks until mid-pregnancy and at least once near 30 weeks, as thyroid function can change quickly. The aim is to keep TSH within the range set for each trimester, to keep risks as low as possible.
Medication timing
Prenatal vitamins containing iron and calcium can interfere with levothyroxine absorption. Take thyroid medication at least 4–5 hours apart from prenatal vitamins.
TSH targets in pregnancy (American Thyroid Association, 2017)
- Normal range: the trimester-specific range from your laboratory; if it has none, an upper limit of about 4.0 mIU/L is used
- If you take levothyroxine: the usual aim is the lower half of that range, or below 2.5 mIU/L if no pregnancy range is available
- Before conception: women on levothyroxine aim for a TSH between the lower reference limit and 2.5 mIU/L
- Always follow your endocrinologist's personalised targets based on your specific labs
Risks of uncontrolled hypothyroidism in pregnancy
Untreated or poorly controlled hypothyroidism in pregnancy can have serious effects on both mother and baby. Knowing these risks shows why good thyroid management matters.
Risks to the mother
Maternal anaemia
Hypothyroidism impairs red blood cell production, worsening pregnancy-related anaemia.
Preeclampsia
Raised blood pressure that can damage organs, linked with uncontrolled thyroid dysfunction.
Miscarriage or stillbirth
Significantly higher rates in women with untreated or poorly controlled hypothyroidism.
Risks to the baby
Premature birth
Thyroid hormone deficiency increases the risk of delivery before 37 weeks gestation.
Developmental delays
Maternal thyroid hormones are essential for fetal brain development, and a shortage can have lasting effects on the child's nervous system.
Low birth weight and growth restriction
A shortage of thyroid hormone can impair fetal growth and lead to intrauterine growth restriction (IUGR).
Why good control matters
Thyroid hormones play a key role in the baby's brain and nervous system development. Keeping thyroid function well controlled during pregnancy can reduce the risk of complications for both mother and baby and support the child's long-term health.
After pregnancy: postpartum thyroiditis
Postpartum thyroiditis is a common but under-recognised condition affecting women in the months following childbirth. It occurs more frequently in women with autoimmune thyroid disease (such as Hashimoto's thyroiditis) or those with elevated TPO antibodies during pregnancy.
When it occurs
Typically begins 2–6 months postpartum, although it can start at any time in the first year, and may persist for several weeks or months. Why: during pregnancy, the immune system is naturally suppressed. After childbirth it becomes active again, which can trigger thyroid inflammation in susceptible women.
The two phases of postpartum thyroiditis
Phase 1
Hyperthyroid phase
Cause: Damaged thyroid releases excess hormones into the bloodstream. Usually transient, lasting a few weeks.
Symptoms:
- Nervousness and irritability
- Rapid or irregular heartbeat
- Sudden weight loss
- Sensitivity to heat
- Fatigue and difficulty sleeping
Phase 2
Hypothyroid phase
Cause: As the thyroid gland becomes depleted, hypothyroid symptoms emerge. Can last months or, in some cases, become permanent.
Symptoms:
- Extreme fatigue
- Sensitivity to cold
- Memory fog and poor concentration
- Muscle aches and constipation
- Reduced milk supply
Breastfeeding and thyroid health after birth
Hypothyroidism after birth can lead to a reduced milk supply. With appropriate hormone replacement therapy, milk production usually returns to normal.
Diagnosing postpartum thyroiditis
Postpartum thyroiditis often goes undiagnosed because its symptoms (fatigue, mood swings) mimic the normal challenges of new parenthood. Diagnosis requires:
TSH and free T4 tests
These show how the thyroid is working and whether you are in the hypothyroid or hyperthyroid phase. They should be done if you have any symptoms after birth.
TPO antibody test
This checks for autoimmune involvement, the main cause of postpartum thyroiditis in most cases.
Treating postpartum thyroiditis
Treatment depends on the phase and how severe the condition is. Most cases need little treatment, and the majority settle on their own.
Treating the hyperthyroid phase
Symptoms are often mild and self-limiting: many women require no specific treatment. If symptoms are disruptive, beta-blockers may be prescribed to manage rapid heartbeat. Note: antithyroid drugs are not typically used, as this phase is due to hormone release, not overproduction.
Treating the hypothyroid phase
Levothyroxine is typically prescribed to normalise thyroid hormone levels. This is especially important if symptoms are severe, the mother is breastfeeding (low supply), or she is planning another pregnancy.
Regular TSH checks
TSH levels should be checked periodically throughout the postpartum year to adjust medication as needed and monitor for transition between phases or permanent hypothyroidism.
- 12–18months for most cases to resolve
- 20%may develop permanent hypothyroidism
- Higherrecurrence risk in future pregnancies
Looking after your thyroid after birth
- Monitor symptoms: be alert to extreme fatigue, mood swings, or trouble breastfeeding
- Schedule regular check-ups, especially if you have a history of thyroid issues or elevated TPO antibodies
- Eat for thyroid support: iodine, selenium, and zinc are the key nutrients
- Work with specialists: an endocrinologist or OB-GYN, as well as your GP
Summary: thyroid care for a healthy pregnancy
If you are planning a pregnancy and have a history of thyroid problems, autoimmune conditions or pregnancy complications, involve your doctor early. They can arrange key tests, including TSH and thyroid peroxidase antibodies (TPOAb), to check your thyroid function and make a personalised plan.
Plan early
Managing your thyroid before, during and after pregnancy lowers the risks and improves your chances of a healthy pregnancy.
Lifestyle and thyroid health
Medical care comes first, but your lifestyle also plays a part in supporting thyroid health and preparing your body for pregnancy.
Nutrition
Eat a balanced diet rich in iodine (iodised salt, dairy, seafood), selenium (Brazil nuts, fish), and zinc (meat, legumes), all essential for thyroid function and hormonal balance. Take iodine or selenium supplements only if your doctor advises them, as too much of either can do harm.
Regular exercise
Engage in moderate physical activity to maintain overall health, regulate hormone levels, and manage the weight gain associated with hypothyroidism.
Stress management
Practise yoga, meditation or deep breathing. Long-term stress raises cortisol, which can affect thyroid function and disrupt reproductive hormones.
Good sleep
Sleep plays a part in regulating thyroid hormones. Poor sleep can worsen hypothyroid symptoms and disrupt the timing of ovulation.
Vitamin D
Vitamin D deficiency is linked with Hashimoto's thyroiditis. Regular sun exposure, and supplements if you are deficient, may reduce autoimmune thyroid activity.
Avoid endocrine disruptors
Reduce exposure to BPA in plastics, pesticides and processed foods, which can interfere with thyroid hormone production and signalling.
Frequently asked questions
Can hypothyroidism cause infertility?
Yes. Hypothyroidism can disrupt ovulation, cause irregular or absent periods, shorten the luteal phase and raise prolactin levels, all of which can reduce fertility. Women with untreated hypothyroidism have significantly lower conception rates. Reaching a euthyroid state (normal thyroid function) before conception improves the chances of a successful pregnancy; for women on levothyroxine, this usually means a TSH between the lower reference limit and 2.5 mIU/L.
What TSH level is safe during pregnancy?
The American Thyroid Association (2017) advises:
- Normal range: the trimester-specific range from your laboratory; if it has none, an upper limit of about 4.0 mIU/L is used
- If you take levothyroxine: TSH in the lower half of that range, or below 2.5 mIU/L if no pregnancy range is available
- Before conception: for women on levothyroxine, a TSH between the lower reference limit and 2.5 mIU/L
TSH should be checked about every 4 weeks until mid-pregnancy and at least once near 30 weeks, as thyroid hormone requirements increase by 25–50% during this period. Your endocrinologist may set personalised targets based on your specific labs and history.
What is postpartum thyroiditis and how long does it last?
Postpartum thyroiditis is inflammation of the thyroid gland that occurs in the months following childbirth, more commonly in women with autoimmune thyroid disease or elevated TPO antibodies. It typically begins 2–6 months postpartum and, in its classic form, passes through a hyperthyroid phase followed by a hypothyroid phase, although many women have only one of the two phases.
Most cases resolve within 12–18 months. However, up to 20% of women may develop permanent hypothyroidism requiring lifelong treatment. Women with a history of postpartum thyroiditis have a higher risk of recurrence in future pregnancies.
Does hypothyroidism affect breast milk supply?
Yes. Hypothyroidism after birth can lead to a reduced milk supply, which adds stress for breastfeeding mothers. However, with appropriate hormone replacement therapy (levothyroxine), milk production usually returns to normal. Levothyroxine is considered safe during breastfeeding and does not harm the baby.
Should I take my thyroid medication differently during pregnancy?
Yes. During pregnancy, these changes are usually needed:
- Take levothyroxine at least 4–5 hours apart from prenatal vitamins containing iron and calcium, which can block absorption
- Your dose will probably need to be increased by 25–50% over the pregnancy, because pregnancy increases the need for thyroid hormone. The American Thyroid Association advises raising the dose by about 20–30% (for example, two extra tablets a week) as soon as pregnancy is confirmed and contacting your doctor straight away for a blood test; agree this plan with your endocrinologist before you conceive
- Get TSH checked about every 4 weeks until mid-pregnancy and at least once near 30 weeks, rather than the usual annual check
References
- Prevalence of hypothyroidism in infertile women and evaluation of response of treatment for hypothyroidism on infertility. Verma I et al. Int J Appl Basic Med Res 2012;2(1):17–19. PMC3657979
- 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Alexander EK et al. Thyroid 2017;27(3):315–389. PubMed 28056690
- Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Stagnaro-Green A et al. Thyroid 2011;21(10):1081–1125 (replaced by the 2017 guideline).
- Postpartum thyroiditis. Lazarus JH, Parkes AB, Premawardhana LD. Autoimmunity 2002;35(3):169–173.
- 2021 European Thyroid Association guideline on thyroid disorders prior to and during assisted reproduction. Poppe K et al. Eur Thyroid J 2021;9(6):281–295.
- Medication timing and levothyroxine absorption. American Thyroid Association. thyroid.org
- Hypothyroidism. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). niddk.nih.gov
Written by
Sarbjeet SinghMSc Dietetics · MSc Chemistry · B.Pharm · Certified Diabetic Educator
Clinical nutrition researcher and registered pharmacist. He leads DietXP's nutrition content and co-authored a 2025 Clinical Nutrition ESPEN study of 1,922 patients at Tata Memorial Hospital.
Medically reviewed by
Charubhala RMSc Clinical Nutrition and Dietetics · Certified Diabetic Educator
Onco-Nutrition Fellow at Tata Memorial Hospital, Mumbai. She checks each Knowledge Hub guide against current clinical evidence and published dietary guidelines.
Disclaimer: This article is for educational purposes only and does not constitute medical or obstetric advice. Hypothyroidism in pregnancy requires specialist management. Always consult your endocrinologist, OB-GYN, or registered dietitian before making any changes to your treatment or diet plan during pregnancy or postpartum.
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