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Originally published on 26th June 2016 Last updated on 10th October 2021
See Also: Preparing for Pregnancy Blood Test
How Can Hypothyroidism Develop During/After Pregnancy?
As pregnancy is stressful on the body, it can induce hypothyroidism. For some women, this starts during pregnancy, but it’s after pregnancy that a lot are diagnosed. Some recover after a month or two, but many are left with hypothyroidism for the rest of their lives, requiring medication.
During pregnancy, the body goes through many hormonal changes and the immune system makes adjustments in order to preserve the foetus and not reject it as a foreign invader.
The Th-1 suppression ends after birth and this causes the immune system to surge. If it is already unstable, then this can trigger the start of Hashimoto’s Thyroiditis.
Hashimoto Thyroiditis is an autoimmune disease where the immune system attacks healthy thyroid gland tissue, slowly destroying it. This often leads to hypothyroidism. Hypothyroidism in pregnancy is usually caused by Hashimoto’s and occurs in three to five out of every 1,000 pregnancies. 
The thyroid gland usually enlarges with pregnancy, but not to a noticeable amount.
Additionally, if you have the autoimmune version of hypothyroidism, you’ll want to keep a check on adequately treating your hypothyroidism and antibodies, since some experts state that thyroid antibodies cross the human placenta and could attack your baby’s thyroid. Research has also shown that high levels of Thyroid Peroxidase Antibodies increase the risk of premature births, so keeping Hashimoto’s well controlled can be crucial. 
See related post: Are Pregnant Women With Thyroid Problems Considered High Risk?
What If I Know I have Hypothyroidism Already but Am Now Pregnant?
As soon as you know you are pregnant, tell your doctor and get a full thyroid panel booked in as soon as possible. That’s a full thyroid panel. Not just TSH and Free T4. Free T3 and thyroid antibodies too. Make it your mission to look after yourself and your baby as well as possible.
In the first trimester of pregnancy, the foetus relies completely on the mother to provide the thyroid hormones for its development. For someone with a perfectly healthy thyroid gland and function, their body is able to meet that extra demand easily. In a woman with hypothyroidism, her body may not be able to.
According to the Endocrine Society’s 2007 Clinical Guidelines for the Management of Thyroid Dysfunction during Pregnancy and Postpartum, thyroid medication usually needs to be increased in dosage, by 4-6 week gestation and may well require a 30-50% increase in dosage.
However, during my pregnancy with hypothyroidism and Hashimoto’s, I didn’t require a dosage increase until the third trimester, so every woman’s needs can differ.
This is likely due to the type of thyroid medication.
After all, a small study from 1990 revealed that although pregnant women taking Levothyroxine (Synthroid) needed an increase in dosage to maintain good thyroid hormone levels in pregnancy, women on NDT medications (such as Armour) did not need to. So this is definitely something to monitor closely and observe in regards to your personal needs. 
If you’re ‘subclinical’ or ‘borderline’ hypothyroid, your doctor may wish to start you on thyroid medication or increase it so that you’re well within range to reduce risk of miscarriage. The risk of miscarriage is higher in women with subclinical hypothyroidism, compared to women with normal thyroid function (euthyroidism). 
Most women with existing hypothyroidism require an increase in thyroid medication when pregnant, to support the developing baby. 
Failure to properly maintain adequate thyroid levels whilst pregnant can result in complications such as miscarriage, pre-eclampsia, anaemia, stillbirth and the baby developing congenital hypothyroidism itself. 
So, it’s very important to be tested regularly, often every 4-6 weeks, throughout your pregnancy. Adjustments to your medication should then be made accordingly.
Something else worth keeping in mind is that HCG, the pregnancy hormone, stimulates the thyroid, often leading to a lower TSH compared to before pregnancy. This is why test ranges change for pregnant women.
Books I found useful in my own thyroid pregnancy:
- Your Healthy Pregnancy with Thyroid Disease
- The Thyroid Hormone Breakthrough
- The Positive Birth Book
- The Pregnancy Encyclopedia
- What to Expect When You’re Expecting
- Healing Your Body Naturally After Childbirth
- The Positive Breastfeeding Book: Everything you need to feed your baby with confidence
- Mindful Hypnobirthing: Hypnosis and Mindfulness Techniques for a Calm and Confident Birth
If you’re struggling with morning sickness interfering with how much of your thyroid medication you absorb, there are some tips listed here.
Thyroid Levels During Pregnancy
The American Thyroid Association (ATA) recommends the use of the following TSH test ranges during pregnancy: 
- First trimester: 0.1-2.5
- Second trimester: 0.2-3.0
- Third trimester: 0.3-3.0
But maintaining optimal thyroid levels for all of the thyroid panel is important, not just TSH.
The UK’s NHS recommends the following: (though the exact numbers can differ slightly depending on your surgery and lab)
(Non-pregnancy range for TSH is usually around 0.5 – 4.4 mu/L)
- 1st Trimester: 0.09 – 2.83 mu/L
- 2nd Trimester: 0.2 – 2.8 mu/L
- 3rd Trimester: 0.31 – 2.9 mu/L
(Non-pregnancy range for Free T4 is usually around 10 -20 pmol/L)
- 1st Trimester: 10.5 – 18.3 pmol/L
- 2nd Trimester: 9.5 – 15. 7 pmol/L
- 3rd Trimester: 8.6 – 13.6 pmol/L
(Non-pregnancy range for Free T3 is usually around 3.5 -6.5 pmol/L)
- 1st Trimester: 3.5 – 6.2 pmol/L
- 2nd Trimester: 3.4 – 5.8 pmol/L
- 3rd Trimester: 3.3 – 5.6 pmol/L
Blogger and thyroid advocate Hypothyroid Mom started her site in memory of the baby she lost due to her thyroid levels not being maintained correctly, and fights to stop this from happening to other women and babies. So maintaining optimal thyroid levels are important.
Some researchers believe that one factor in the development of autism is severe hypothyroidism in their mothers. 
Mental Health and Pregnancy
Although controversial, I’m going to say it: postnatal depression could actually be due to low thyroid function.
So if you have been told you have postnatal depression, it is definitely worth having a full thyroid panel tested to check your thyroid. In fact, I feel that thyroid screening should become mandatory during all pregnancies. Whether you’re diagnosed with a thyroid condition or not, I believe that thyroid levels should be monitored. As explained above, some women develop thyroid problems during pregnancy.
I say it’s controversial to claim that some of the postnatal depression cases out there could be due to poor thyroid function, because some would suggest I am implying that postnatal depression isn’t a real thing in its own right. But it is, I acknowledge that. What I’m saying is that the thyroid could be responsible for at least some of these cases, because of its link to mental health. You can’t argue that this can’t be true, when low thyroid function is proven to be linked to poor mental health. There will be women out there who have been diagnosed with postnatal depression, have hypothyroidism, and have no idea. Only testing a full thyroid panel, definitely checking the all important Free T3, will rule this out.
If someone has postnatal depression after pregnancy, then there is a chance they may have developed Hashimoto’s and/or hypothyroidism, which is causing the symptoms.
The link is thought to involve T3, one of the hormones a healthy thyroid should be producing. T3 is the most active thyroid hormone and required for a lot of things.
T3 has an important role in the health and optimal functioning of your brain, including: your cognitive function, ability to concentrate, mood, memory and attention span and emotions and ability to cope with life’s stresses. T3 interacts with brain receptors and makes the brain more sensitive to chemicals such as Serotonin and Norepinephrine, which affects your alertness, memory, mood and emotion.
So if your doctor failed to check your thyroid levels with the correct thyroid tests (TSH, Free T3 and Free T4), when you showed signs of poor mental health, then you may have mental health issues caused by an inadequately treated thyroid problem, most likely a low T3. So postnatal depression could actually be caused by thyroid levels being out of whack.
If you have had your thyroid levels checked and been told they’re fine, make sure they are in fact optimal. It can make a lot of difference.
Postpartum Thyroiditis is where the thyroid gland becomes inflamed after pregnancy, and this happens to about 5-7% of women, usually within a few months of giving birth. Interestingly, this is also a form of autoimmune disease. , 
This usually presents as a painless, small enlargement of the thyroid, and can cause either hyperthyroid or hypothyroid symptoms.
This can also lead to postpartum depression because of its impact on thyroid function.
Similar to subacute thyroiditis, there are potentially two phases to postpartum thyroiditis. The inflammation and release of thyroid hormones in to the blood usually first causes symptoms of hyperthyroidism:
- Increased sensitivity to heat
- Rapid heartbeat or palpitations
- Unexplained weight loss
These usually occur within a few months of giving birth. It is important to figure out if it’s postpartum thyroiditis or Graves’ disease with proper testing.
As thyroid cells are continually attacked, signs and symptoms of hypothyroidism can develop:
- Lack of energy/fatigue/weakness
- Increased sensitivity to cold/cold hands and feet
- Dry skin
- Difficulty concentrating/brain fog/confusion
- Aches and pains
Most women who experience postpartum thyroiditis return to normal thyroid function after about a year, however, around a third develop permanent hypothyroidism. , 
What has been your experience of pregnancy and hypothyroidism?
If you’re looking for an online support group, I came across this one: Pregnancy and Hypothyroidism on Facebook. Please note that I am not a member or admin.
You can click on the hyperlinks in the above post to learn more and see references to information given.
You, Me and Hypothyroidism: When Someone You Love Has Hypothyroidism, a book for those who know someone with hypothyroidism. It looks at how pregnancy, fertility, parenting, home life and more can all be affected by hypothyroidism, and what you can do about it.
Rachel Hill is the highly ranked and multi-award winning thyroid patient advocate, writer, speaker and author behind The Invisible Hypothyroidism. Her thyroid advocacy work includes writing articles, authoring books, producing her Thyroid Family email newsletters and speaking on podcasts, as well as being a founding board member for the American College of Thyroidology. She is well-recognised as a crucial and influential contributor to the thyroid community and has a large social media presence. Her books include “Be Your Own Thyroid Advocate” and “You, Me and Hypothyroidism”.