The Pregnancy – Hypothyroid Connection

This post focuses on the importance of sufficient thyroid hormones during pregnancy, and also touches on Hashimoto’s hypothyroidism related to pregnancy. Read about hypothyroid and fertility here.

Your thyroid naturally undergoes changes during pregnancy. Not only does your hormone production change, but your actual thyroid gland also changes too, it gets larger! This is because during pregnancy your need for thyroid hormones (T4 and T3) increases. You need sufficient thyroid hormones for metabolism and growth, and these demands increase while you are growing a new human being. Therefore, it’s a good idea to test your thyroid hormones before conception to get a baseline. It’s also important to test throughout your pregnancy. 

Understanding TSH

When the demand for more thyroid hormones goes up, your pituitary gland secretes TSH (thyroid-stimulating hormone), which tells your thyroid to make more hormones. Then once you make enough T4 and T3, TSH lowers. This endocrine feedback loop normally works well during pregnancy, given that there are no obstacles.

The caveat about testing TSH is that the lab reference ranges are outdated, Even for women who are not pregnant. When you are pregnant, your doctor needs to pay attention to accurate ranges for pregnancy. In 2011 the American Thyroid Association (ATA) recommended an upper TSH limit of 2.5 mU/L in the first trimester, and 3.0 mU/L in the second and third trimesters. In 2017, the ATA guidelines for TSH became more detailed, taking into account previous thyroid illness, iodine intake and different patient populations.

ATA recommendations for TSH: Upper limit of 2.5 during the first trimester and 3.0 during the rest of the pregnancy. Functional medicine recommendations: Upper limit of 2.5 during the first trimester and 2.5 during the rest of the pregnancy

Understanding T4

If you’re interested in exactly how much more T4 you need during pregnancy, here’s how it works: T4 levels first increase at week 7, then continue until week 16. After week 16, levels remain approximately 50% higher than pre-pregnancy levels.

However, the lab reference ranges don’t change for pregnancy. So if you want to monitor your levels, after week 16 you adjust the upper lab range to 50% higher. Starting with week 7, adjust the upper lab range 5%, and add another 5% each week.

Here’s how to adjust the upper range of T4 for weeks 7 – 16:

  • 7th week: 5% higher
  • 8th week: 10% higher
  • 9th week: 15% higher
  • 10th week: 20% higher
  • 11th week: 25% higher
  • 12th week: 30% higher
  • 13th week: 35% higher
  • 14th week: 40% higher
  • 15th week: 45% higher
  • 16th week: 50% higher

Now why am I giving you this much detail about TSH and T4 levels? Simply because some doctors in the USA do not routinely test thyroid during pregnancy – yet. This may be something you need to ask for, and have the knowledge to analyze the results yourself.

Hypothyroidism is when TSH is high (the message is please make more thyroid hormone!) and T4 is low. Let’s talk about why it’s important to avoid hypothyroidism during pregnancy…

Hypothyroidism during pregnancy

When its overt (both TSH and T4 out of the lab ranges above), it:

When it’s subclinical (TSH high and T4 normal):

  • Miscarriage
  • Preterm labor
  • Possible cognitive impairment (this is inconclusive in studies, but enough evidence suggests it should be a concern)

So there are real and serious repercussions of hypothyroidism during pregnancy. Fortunately, it’s easy to treat. Sometimes it’s just a matter of nutrient deficiency or supplementation with thyroid hormone by prescription. For many women, this is only needed during pregnancy and not afterward. Learn about all the thyroid nutrients in this post.

You need sufficient iodine!

Iodine is an essential building block for T4. Since you need more T4 during pregnancy, you also need more iodine. Secondly, you actually excrete more iodine during pregnancy. Thirdly, your developing fetus needs iodine. Therefore, you need more iodine during pregnancy.

The U.S. Institute of Medicine advises pregnant women get 220 µg iodine, and breast-feeding women get 290 µg. Check out iodine sources in this post. Note that if you are on T4 hormone replacement, such as levothyroxine, you do not require iodine supplementation, because the medication bypasses your need for it. Also, kelp as a food, or in prenatal vitamins, is an unreliable source of iodine due to large fluctuations in content.


Before trying to conceive, the ATA recommends women supplement their diet with 150 μg iodine, as oral potassium iodide. Ideally, start 3 months in advance of pregnancy.

What happens with iodine deficiency?

Your thyroid gland can enlarge 10-15% during pregnancy. However, with iodine deficiency, this percentage may increase, and you may develop an iodine-deficient goiter. Many other problems will occur with mild, moderate, or severe iodine deficiency, for both mother and baby.

You may think women in the U.S. are not iodine deficient. However, the 2005–2010 National Health and Nutrition Examination Survey (NHANES) surveys showed the median U.S. pregnant woman had mild iodine deficiency.

MILD iodine deficiency can cause:

SEVERE iodine deficiency is much worse and can lead to:

  • For mother: thyroid nodules, goiter.
  • For baby: goiter, cretinism (profound intellectual impairment, deaf-mutism, motor rigidity), stillbirth, and miscarriage (1, 2, 3, 4).

Iodine deficiency is the leading cause of preventable intellectual deficits worldwide. Iodine supplementation can help reverse deficiency, but only if taken before pregnancy or very early on. This paper reports That is iodine supplementation starts 10 to 20 weeks into the pregnancy, it may be too late to correct the deficiency – if the deficiency existed before pregnancy.

What about Hashimoto’s?

Hashimoto’s thyroiditis is when your hypothyroidism is due to an autoimmune disease. First of all, this is more of an immune condition rather than a thyroid condition. With Hashimoto’s, when it’s not in remission, your thyroid is under attack. During this attack, your thyroid releases surges of hormones, which can cause wildly varying levels of thyroid hormones and TSH. It’s a great idea to get Hashimoto’s into remission before pregnancy.

After that, it’s important to test TSH at the very least, every four weeks. Studies show that about 20% of pregnant women with these antibodies have TSH >4 mU/L (too high!). That’s why you test TSH every four weeks during your pregnancy. 1, 2

Increased risks correlated with Hashimoto’s:

  • Greater miscarriage risk
  • Increased rate of preterm delivery

Treatment with T4 for prevention

The ATA does not recommend treatment with T4 to prevent miscarriage or preterm delivery, simply because of not enough evidence. However, there is evidence that T4 may help prevent recurrent pregnancy loss. Many doctors do use T4 to try preventing miscarriage and preterm delivery.


  1. Get T4, TSH, and thyroid antibodies measured before pregnancy. If antibodies are positive, you can get on a remission plan.
  2. If you show up with overt or subclinical hypothyroidism, talk to your practitioner about treatment options. Test the nutrients associated with thyroid function, and replenish ideally before conception – especially iodine.
  3. If Your thyroid levels are normal before conception or during pregnancy, it’s prudent to test thyroid levels throughout pregnancy, as your thyroid does go through special changes and demands.

Read about hypothyroid, periods, and fertility here.

Please ask questions or give feedback below,

paris healing arts, doctor laura paris, dr. laura paris, dr laura paris

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One Comment

  1. Very nice informative and impressive post you have written, this is quite interesting and i have went through it completely, an upgraded information is shared, keep sharing such valuable information.

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