How can thyroid health affect fertility and pregnancy?

The thyroid helps regulate metabolism and a whole host of hormonal mechanisms. So, what effect does a thyroid condition have on fertility and pregnancy?

Having an undiagnosed thyroid condition can affect fertility and may cause complications during pregnancy. And for women with a pre-existing thyroid condition, thyroid hormones need to be tightly regulated to support the baby’s development.   

But the good news is, once detected, thyroid conditions can be well controlled, and with appropriate care, most women go on to have a healthy pregnancy.  

In this blog, we go on to explain: 

What role does the thyroid play in fertility?

The thyroid gland helps to regulate the body’s metabolism, growth, and development. But it also plays a role in reproduction, regulating the menstrual cycle and ovulation. Or in men, sperm production.  

How can a thyroid condition affect female fertility?

For conception to occur, an egg must first be released from the ovary (ovulation) and then be fertilised by a sperm cell. Thyroid conditions can make it more difficult to conceive, as an imbalance in thyroid hormone levels may mean ovulation does not occur (even if a woman still has periods).   

How can a thyroid condition affect male fertility?

Thyroid hormone imbalances in men can affect the quality of sperm. They may not produce enough sperm (resulting in a low sperm count), or sperm may not form correctly or behave in the right way for fertilisation.  

Planning a pregnancy (pre-conception) with a thyroid condition

The good news is, whether you have an underactive or overactive thyroid, once the condition is managed and hormones return to a normal level, ovulation usually returns. Sperm count and quality will usually recover too. That means most couples will be able to conceive and have a healthy pregnancy if their condition is well managed.   

If you have an underlying thyroid condition and are planning a pregnancy, it’s important to see your doctor as soon as possible. You may also need a referral to see an endocrinologist.   

They'll carry out regular monitoring during your pregnancy to keep you healthy and prevent any relapse of any thyroid condition.   

Some medicines used to treat an overactive thyroid are not suitable to take during pregnancy as they can harm a developing baby. So, you will usually be switched to a more suitable medication, and it is best to do this before pregnancy.   

Pregnancy and thyroid conditions

During normal pregnancy, the thyroid gland will get slightly larger (hypertrophy) as thyroid cells increase in size.   

Pregnancy can reveal or unmask an underlying thyroid condition that was already present but previously was not causing noticeable problems.   

Sometimes, the stress of pregnancy on the body can trigger an overactive or underactive thyroid condition (usually because of an underlying autoimmune disease).    

If left uncontrolled, an overactive or underactive thyroid can increase the chances of pregnancy complications.  

An uncontrolled thyroid condition can increase the chances of:  

  • Pre-eclampsia  
  • Heart failure  
  • Premature birth  
  • A low birth weight in baby   
  • Problems with the baby’s brain development   
  • The risks of a miscarriage or stillbirth 

How does pregnancy affect my thyroid hormones?  

The gland will need to change the level of hormones to meet the metabolic demands of pregnancy. 

This will lead to changes in hormone levels: 

Symptoms of low and high t4 levels table

  • TSH – HCG stimulates the thyroid-stimulating hormone (TSH) receptor, which leads to a mild fall in TSH in the 1st trimester. TSH increases to near-normal levels for the remainder of the pregnancy. Note that women with an underactive thyroid may have slightly lower target levels of TSH during pregnancy. 
  • Total T4 – Thyroid binding globulin increases in pregnancy which leads to higher levels of total T4. 
  • Free T4 – There is a transient rise in free T4 in the first semester. with a modest decrease in the second and third semester. Free T3 follows a similar pattern.   

Can a thyroid condition cause recurrent miscarriages?

Undiagnosed or uncontrolled health conditions in the mother is associated with recurrent miscarriage. And some studies show that having thyroid antibodies are among these associated risks for miscarriage. However, the studies are mixed.  

It is less clear whether any treatment is needed for someone whose thyroid is healthy, but they have antibodies. If you are at risk of thyroid conditions or are concerned about whether you have thyroid antibodies, it is worth getting a thyroid test or consulting your doctor.   

Will I be screened for a thyroid condition when trying for a baby?

While routine screening of women who are planning a pregnancy, or are already pregnant, is not currently recommended in the UK, there are several situations where NICE does recommend screening blood tests to check for an underactive thyroid.   

Generally, routine screening for an underactive thyroid in pregnancy is considered if the woman: 

  • Is over 30 years old  
  • Has a goitre  
  • Comes from a region in the world with high levels of iodine deficiency  
  • Has a previous history of a thyroid condition  
  • Has a family history of thyroid disease  
  • Has type 1 diabetes or other autoimmune condition  
  • Is known to be positive for thyroid antibodies  
  • Has a history of infertility, miscarriage, previous premature delivery  
  • Is obese, with a BMI greater than 40  

If you are worried you may have an undiagnosed underlying thyroid condition, Medichecks also offer tests for thyroid hormones and thyroid antibodies.   

You can find out more in our Thyroid Blood Test Buying Guide.   

How is an underactive thyroid condition managed during pregnancy?

Women with an underactive thyroid will need more than their usual levels of thyroxine to keep their hormone levels within the desired range. Usually, a slightly lower TSH (by increasing the thyroxine dose) will be recommended while trying for a baby. This is because adequate thyroid hormone levels are crucial in the very earliest weeks of pregnancy to ensure the healthy development of the baby’s brain and nervous system.   

Until around 12 weeks, the baby will rely on Mum’s supply of thyroid hormones. Beyond this age, the baby’s thyroid will start to produce low levels of hormones itself, but amounts are insufficient until around 18-20 weeks.   

Target thyroid hormone levels will change depending on the stage of pregnancy or trimester, and TSH and T4 levels should be checked at least every eight weeks.   

After delivery of the baby, levothyroxine doses should be reduced to pre-pregnancy levels, and thyroid-stimulating hormone levels tested around six to eight weeks later.  

How is an overactive thyroid condition managed during pregnancy?

For pregnant women with an overactive thyroid, or those hyperthyroid women planning a pregnancy, medication is usually the best option. Any woman taking carbimazole will usually be switched to an alternative called propylthiouracil which is thought to be safer for the baby in the early stages of pregnancy. However, propylthiouracil can cause maternal side effects including liver problems, so the current best practice is to switch back to carbimazole as the pregnancy progresses.  

Thyroid conditions and post-pregnancy

Babies born to mothers taking antithyroid medication can develop an underactive thyroid, so they must have their thyroid function checked shortly after birth and again a few days later. Mums will also need to have their thyroid hormone levels checked six weeks after giving birth and in a further six weeks.   

Breastfeeding is still safe while on thyroxine. For women on antithyroid medication, little of the medication is thought to be present in breastmilk so breastfeeding is still considered to be safe, however, the baby’s thyroid hormone levels should be regularly checked to ensure levels are remaining within normal ranges.  

Some women will develop thyroid disease within a year of delivering their baby and this is known as post-partum thyroid disease. As before, there is usually an autoimmune cause, and women with type 1 diabetes are particularly prone to developing post-partum hyperactive thyroid disease.   

Symptoms of underactive and overactive thyroid disease can be vague, and difficult to distinguish from normal symptoms of childbirth and having a new baby. However, it’s worth checking for thyroid disease if symptoms are persisting, and it’s something to mention at your routine six-week check-up with your doctor after having a baby.   

For more information on fertility or your thyroid, visit our hubs or take a look at our Female Fertility Tests and Thyroid Blood Tests


  1. Tidy, D., 2022. Thyroid Disease In Pregnancy. Pregnant Thyroid Symptoms. [online] Available at: <> [Accessed 1 July 2022].   
  2. Newson, D., 2022. Hyperthyroidism in Pregnancy. Medical information. [online] Available at: <> [Accessed 1 July 2022]. 
  3. 2022. Scenario: Pre-conception, pregnancy, and postpartum | Management | Hyperthyroidism | CKS | NICE. [online] Available at: <> [Accessed 1 July 2022].  
  4. 2022. Context | Thyroid disease: assessment and management | Guidance | NICE. [online] Available at: <> [Accessed 1 July 2022]. 
  5. 2022. Overactive thyroid (hyperthyroidism) - Complications. [online] Available at: <> [Accessed 1 July 2022].  
  6. 2022. Overactive thyroid (hyperthyroidism) - Treatment. [online] Available at: <> [Accessed 1 July 2022]. 
  7. British Thyroid Foundation. 2022. Pregnancy and fertility in thyroid disorders. [online] Available at: <> [Accessed 1 July 2022]. 
  8. Pregnancy, T. and Health, N., 2022. Thyroid Disease & Pregnancy | NIDDK. [online] National Institute of Diabetes and Digestive and Kidney Diseases. Available at: <> [Accessed 1 July 2022]. 

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