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Thyroid in pregnancy: hypothyroid, hyperthyroid and what we can do about it.

postpartum pregnancy May 20, 2020

Thyroid in Pregnancy:

Thyroid problems in pregnancy affect about 3-5% of women in Canada.  

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Thyroid hormone is produced in the thyroid and is known as T4.  T4 is then released into the bloodstream and is turned into T3, which is the active form of the hormone, in the organs which use it, including the thyroid.   


Both T4 and T3 are bound by proteins in our blood, and only a very small amount of it is unbound or Free in our blood.  This is so that if our levels dip, hormones can quickly be released to normalize it again.   


Thyroid hormones, in adults, have a big influence on our metabolism.  In fetuses, thyroid hormones play an important role in brain development.  


Thyroid hormones are made up of quite a bit of iodine, and so it is important in pregnancy to increase foods rich in iodine.  These include seaweed, eggs, cod, shrimp, Lima Beans, dairy and Prunes - which added bonus help with constipation!! 


The thyroid glands production of T4 is controlled much like a thermostat in our house controlling heat…if it is too hot the thermostat recognizes this and turns the furnace off, and if it is too cold, then it turns the furnace on.  The same thing happens with the thyroid.  Our pituitary gland acts like the thermostat and if it recognizes that our thyroid hormone levels are too low, it will release a hormone called thyroid-stimulating hormone which, you guessed it, stimulates the thyroid to produce more T4.  If we have too much hormone in our body then the amount of TSH produced is decreased and the production of the thyroid hormones slows. 

There are two main issues that can happen with your thyroid:


  • Our thyroid hormones can be overproduced causing hyperthyroidism
  • Our thyroid hormones can be underproduced causing hypothyroidism. 


Both of these issues can have impacts on the health of our pregnancy both early in pregnancy and later on.  They can both have effects on our baby as well, so it is important to recognize them and treat them appropriately. 


Now let’s talk about thyroid hormones specifically in pregnancy: 


Our body naturally starts ramping up the production of thyroid hormones early in pregnancy, around 5-6 weeks, and by 20 weeks it is producing almost 50% more hormone then when it is not pregnant.  This is because our baby depends on our thyroid hormone (T4) as it does not start producing its own until midway through the pregnancy.  


Our pregnancy hormone, Bhcg, also mimics TSH in early pregnancy and causes an increase in thyroid hormones.  Because the bHCG is ramping up production of hormones the pituitary gland can decrease the production of TSH which can make our blood work appear wonky…but this is totally normal.  We often have a very low TSH (which seems like hyperthyroidism) but normal thyroid hormones. We just need to monitor this to make sure it normalizes after 20 weeks.  


Thyroid issues that can come up in pregnancy!  


Hyperthyroidism in Pregnancy:


Hyperthyroidism in pregnancy is relatively rare in its true form in pregnancy.  It can often appear to be present in the first half of pregnancy, because the pregnancy hormone looks similar to TSH in our body, so the amount of TSH decreases significantly but the levels of T4 and T3 are normal.  This is not true hyperthyroidism so just needs to be monitored.  This generally returns to normal in the second half of pregnancy, and is not dangerous to mom or baby.  


True Hyperthyroidism results in the thyroid hormones T3 and T4 being too high and as a result the TSH being very low.  If this is the case then we need to treat mom to protect her and her baby.  If there are issues with hyperthyroidism in pregnancy, we need to monitor mom and baby carefully postpartum. 


Hypothyroidism in Pregnancy: 


The more common issue in pregnancy is hypothyroidism, or low levels of the thyroid hormones T3 and T4 and as a result high levels of TSH.  Because we know that moms and their babies need higher levels of the hormone to support the pregnancy we are much more cautious about TSH levels in pregnancy, and treat sooner than if you were not pregnant.  


The main types of hypothyroidism are 

  1. that caused by not enough iodine intake. 
  2. Autoimmune - otherwise known as Hashimoto’s thyroids (which sounds very scary!!) is the most common in societies with iodine supplementation in their salt.
  3. There are other rare causes of hypothyroidism, but these are very uncommon 


Why do we care?  


Women with hypothyroidism in pregnancy are at a higher risk of complications of pregnancy such as high blood pressure in pregnancy, placental abruption and pregnancy loss.  Now let's keep this in perspective, these are not huge risks, but if we can decrease those risks why wouldn’t we?  We also know that fetal brain development depends on appropriate levels of thyroid hormones, and we also want to ensure we are giving our baby the best start to life!


So what do we do? 


Here in Victoria we screen all pregnant women early in pregnancy to see what they TSH (thyroid-stimulating hormone) levels are.  If with this screening we diagnose them with hypothyroidism, we discuss replacing their low thyroid hormones with levothyroxine which is a synthetic T4 to normalize their hormone levels. 


If you are planning on becoming pregnant or newly pregnant, make sure to check our free preparing for pregnancy guide found at to get yours.


Desiccated Thyroid Hormone:


Now some people may ask what about desiccated thyroid?  Well we know that it comes from pigs and is a hormone that is dried into a powdered form that is then bound together by chemicals.  It also is not consistent from batch to batch, so challenging to properly maintain consistent dosing.  Animals also have different ratios of T4 and T3 so it is not consistent with human physiology.  If we simply supplement with T4 the body is able to convert it in a way that is physiologically consistent with our natural T4/T3 levels. 


We also know that most of the T3 or active thyroid hormone in our brains comes from T4 which is converted in the brain…ie the t4 crosses from the blood into the brain and is then converted to the active hormone.  The ability of T3 to cross over from the blood to the brain is very low and needs extremely high doses.  As a result if you are not supplementing with adequate doses of T4 your baby will not get the active hormone in his or her brain and it will affect their brain development. 


How do we monitor Hypothyroidism in pregnancy? 


If we diagnose hypothyroidism we then follow the hormone levels closely to keep them within a normal level throughout the pregnancy.  We check the hormone levels every 4-6 weeks until normalized and then once a trimester after that. 


For women who are already taking levothyroxine prior to pregnancy, we will often increase their dose with a positive pregnancy test, or if you needed it in your last pregnancy, but not in between pregnancies, then we will start it with a positive pregnancy test as we know those needs increase around 5-6 weeks. 


What about after pregnancy? 


If you were hypothyroid prior to pregnancy we will transition you back to your pre-pregnancy dose of levothyroxine.  If you were not hypothyroid prior to pregnancy, we will over 6-12 weeks wean you off of your dose and monitor your thyroid levels.  There is a risk of becoming hypothyroid ongoing and so it is a good idea to monitor your thyroid levels semi-regularly (maybe once a year or if you have symptoms) if you had thyroid issues during your pregnancy. 


In Recap:


  • Thyroid issues are one of the more common complications of pregnancy, but for the vast majority of women are easily managed. 


  • We screen all women in early pregnancy to make sure TSH is normal. 


  • If we need to treaty hypothyroidism we use Levothyroxine, which is a synthetic T4, so it can get to all the tissues in mom and baby to be changed to the active form T3 where and in what quantities it is needed. 


  • If you have hypo or hyperthyroidism you should be monitored after pregnancy. 


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