Fertility, thyroid, and pregnancy

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 it isn’t all doom and gloom. The good news is, that once detected, thyroid conditions can be well controlled, and with appropriate care, most women can go on to have a healthy pregnancy. 

In this blog, we go on to explain:

What is the thyroid gland?

The thyroid gland sits at the front of the neck and cannot usually be seen or felt. Conditions with the thyroid gland can result in a goitre (a noticeable swelling of the thyroid gland that can be seen and felt in the throat area). 

Thyroid hormones affect cells and organs in the body. They play a part in regulating metabolism - the rate at which your body uses energy. If too much hormone is produced (overactive thyroid), the cells work too quickly. If not enough thyroid hormone is produced (underactive thyroid), the body’s cells slow down, and processes work more sluggishly. 

What is an overactive thyroid?

An overactive thyroid, also known as hyperthyroidism, occurs when the thyroid produces high levels of thyroxine, which reduces thyroid-stimulating hormone levels.  

Symptoms of an overactive thyroid include: 

  • Feeling edgy or anxious 
  • Weight loss 
  • Tremors 
  • Weakness 
  • Sensitivity to heat  
  • Diarrhoea  

Around 0.75% of the population will have an overactive thyroid condition, and women are affected more commonly than men. It is usually caused by an autoimmune condition called “Graves’ Disease”, where the body’s immune system causes the body to produce too much thyroid hormone. An overactive thyroid is usually treated by a doctor who specialises in hormones (an endocrinologist). In the UK, treatments are generally successful at managing the condition. 

For more on an overactive thyroid, have a look at our blog about what is an overactive thyroid?  

What is an underactive thyroid?

An underactive thyroid, also known as hypothyroidism, occurs when the thyroid produces little thyroxine.  

Hypothyroidism is found in around 2% of the UK population and affects women more often than men.  

Symptoms of an underactive thyroid include:  

  • Tiredness 
  • Weight gain 
  • Feeling low and depressed 
  • Constipation 
  • Dry skin and hair 
  • Sensitivity to cold 
  • Irregular and/or heavy periods 

An underactive thyroid may be caused by another autoimmune condition called Hashimoto’s disease, where the body produces antibodies that attack the thyroid gland and affect its ability to produce sufficient levels of thyroid hormones. 

Managing an underactive thyroid is usually done by supplementing daily thyroid hormone tablets. This is treated under the care of your GP or sometimes by an endocrinologist.  

For more on an underactive thyroid, have a look at our blog about what is an underactive thyroid? 

How does a thyroid condition affect fertility?

As well as affecting energy and metabolism, imbalances in thyroid hormones can also affect 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).  

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)

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. And sperm count and quality will usually recover. That means most couples will be able to conceive and have a healthy pregnancy.  

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 careful monitoring and increased checks during your pregnancy to keep you healthy and prevent relapse of any thyroid condition.  

Some medicines 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.  

During pregnancy

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

The gland needs to produce 50% more hormone during pregnancy to maintain healthy normal levels. This will usually lead to increases in the amount of total thyroxine and T3 - although free thyroxine and free t3 remain within normal range, and levels of thyroid-stimulating hormone levels remain largely the same.  

Pregnancy and thyroid conditions

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).   

Facts on pregnancy and thyroid: 

  • Types of underactive thyroid will occur in as many as 2.5% of pregnant women in the UK 
  • Overactive thyroid is much less common and will occur in around two in 1,000 pregnancies in the UK 
  • A type of transient thyroid disorder can occur during early pregnancy, and this is associated with high levels of circulating hCG. This tends to improve as the pregnancy progresses and levels of hCG stabilise. 

Thyroid symptoms can mimic the symptoms of a normal pregnancy. So, if you develop any persistent symptoms of under or overactive thyroid disease, investigate them with blood testing to check thyroid hormone levels.  

Anyone newly diagnosed with a thyroid condition during pregnancy will need a referral to an endocrinologist. 

It’s important to detect a new thyroid condition and properly manage pre-existing thyroid conditions during pregnancy. 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

Thyroid conditions and recurrent miscarriage 

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.  

Routine screening for thyroid conditions in the UK

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.  

When is routine screening for an underactive thyroid recommended in the UK?  

In general, 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 to test for thyroid hormones and thyroid antibodies.  

You can find out more in our Thyroid Test buying guide.  

What happens if I have a thyroid condition and get pregnant?   

Women with an underactive thyroid will need more than their usual levels of thyroxine to keep their hormone levels in the desired range. Sometimes a slightly higher level 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 thyroid hormones supply. 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 thyroid-stimulating hormone 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-eight weeks later. 

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 to carbimazole as the pregnancy progresses. 

After the baby is born

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 hub sections or take a look at our Female Fertility Tests and Thyroid Blood Tests.


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