Your thyroid gets extra busy during pregnancy, and yet this small gland is easy to forget. I find that a shame, because a slow or fast thyroid can affect how you feel and sometimes the course of your pregnancy. Clear information about the TSH value helps you understand a result better.
This article is part of our series on blood testing in pregnancy and zooms in on the thyroid.
Below you will read why your thyroid works harder, what a TSH value roughly shows and when testing can be useful. If you first want the complete overview, read our article on blood tests during pregnancy. Remember that your doctor always assesses your result together with your symptoms.
Why does your thyroid work harder during pregnancy?
Your thyroid works harder in pregnancy because your body seems to need more thyroid hormone, especially in the first trimester. The pregnancy hormone hCG resembles TSH in structure and can stimulate your thyroid further. As a result your TSH value often comes out a bit lower at the start than outside pregnancy.
Your thyroid makes hormones that regulate your metabolism, and in the first months your baby depends entirely on your thyroid hormone. Only later in pregnancy does your baby's own thyroid start to work. That is why a well-working thyroid can be extra important in this period.
Your iodine needs also rise, because your thyroid needs iodine to make hormone.
What is a normal TSH value during pregnancy?
A normal TSH value in pregnancy usually sits a bit lower than outside it, especially in the first trimester. Many labs and guidelines use a rough upper limit of around 2.5 to 3.0 mE/l per trimester. The exact limit differs per lab and per measurement method, so your doctor always assesses your result themselves.
The table below gives indicative TSH reference values per trimester. See it as a rough guide and not as a diagnosis, because the numbers differ per lab and per assay. Your midwife, GP or gynaecologist links your value to your situation.
| Trimester | Indicative TSH (mE/l) | Note |
|---|---|---|
| First trimester | around 0.1 to 2.5 | Often a bit lower due to the hCG effect |
| Second trimester | around 0.2 to 3.0 | Values rise slightly towards normal |
| Third trimester | around 0.3 to 3.0 | Differs per lab and per measurement method |
Values just above or below these limits do not immediately mean something is wrong. Sometimes the free T4 is measured alongside TSH, to get a more complete picture. What your value means is something you discuss with your doctor.
Slow thyroid (hypothyroidism) and pregnancy
A slow thyroid (hypothyroidism) means your thyroid makes too little hormone, which often goes together with a raised TSH value. In pregnancy an untreated slow thyroid can, according to Thuisarts.nl, affect your symptoms and possibly the pregnancy. Treatment is usually with thyroid hormone in tablet form.
Symptoms like tiredness, feeling cold quickly or low mood can fit a slow thyroid, but also simply belong to pregnancy. So a single symptom is rarely enough to say anything about your thyroid. A blood test can help to get more clarity.
Women who already use thyroid hormone sometimes need a higher dose in pregnancy, but your doctor decides that.
Fast thyroid (hyperthyroidism) and pregnancy
A fast thyroid (hyperthyroidism) means your thyroid makes too much hormone, which usually goes together with a lowered TSH value. A slightly lowered TSH is often normal in the first trimester due to the hCG effect. A strongly lowered value with symptoms can be a reason for further testing.
Symptoms that can fit a fast thyroid are palpitations, sweating a lot, restlessness or losing weight while you eat normally. These symptoms also overlap with ordinary pregnancy signs. So your doctor looks at the whole picture and not at one number.
When do you get your TSH tested?
Your TSH is not tested as standard in every pregnant woman, but it is when there is a reason. According to the NHG guideline on thyroid disorders (Schildklieraandoeningen) and Thuisarts.nl, that can be, for example, with a known thyroid condition, earlier thyroid problems or matching symptoms. Your midwife or GP decides together with you whether testing is useful.
With repeated miscarriages or trouble getting pregnant, the thyroid is sometimes included too. You can find more on this value on our page about TSH (thyroid-stimulating hormone). If you have doubts about other values, also look at our article on the gestational diabetes test and blood values.
Testing your own thyroid
Blood testing in pregnancy runs through your midwife or GP. If you also want to map your thyroid yourself, for example before you get pregnant or between check-ups, you can do so at Lunara without a referral. The Thyroid Complete test looks at your TSH among other values.
I always advise you to discuss an abnormal result with your midwife or GP. Testing yourself does not replace the regular pregnancy check-ups. See it as an addition and as a starting point for a conversation with a professional.
Frequently asked questions
Is TSH a standard part of pregnancy testing?
No, TSH is not part of the standard blood testing offered to every pregnant woman. It is usually only tested on indication, for example with a thyroid condition or matching symptoms. Your midwife or GP decides whether it is useful for you.
What does a high TSH in pregnancy mean?
A high TSH can point to a slow thyroid, but one value is rarely enough for a conclusion. The limit differs per lab and per measurement method, and sometimes the free T4 is measured too. So always discuss a raised result with your doctor.
Can a thyroid problem affect getting pregnant?
A thyroid that is out of balance can, according to Thuisarts.nl, sometimes be linked to problems around your period or fertility. Whether this plays a role for you cannot be said from one test. With a wish to conceive and symptoms, your GP can think along about which testing fits.
References
- NHG. NHG guideline on thyroid disorders (Schildklieraandoeningen). Dutch College of General Practitioners.
- Thuisarts.nl. I am pregnant and have a thyroid condition. Thuisarts.nl (NHG).
- RIVM. Prenatal screening for infectious diseases and erythrocyte immunisation (PSIE). National Institute for Public Health and the Environment.
- International guidelines for thyroid function in pregnancy use indicative TSH reference values that differ per trimester and per assay.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. Blood testing through Lunara does not replace the regular pregnancy check-ups. For treatment decisions, discuss your results with your GP or midwife.
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