A well-functioning thyroid is essential for not only a successful pregnancy but a healthy postpartum period for you and your baby. The thyroid is a small, butterfly-shaped gland found in the neck at the side of the throat. Essentially, it’s a powerhouse when it comes to regulating your metabolism and your hormones, so pregnancy, with all its changes to the body, can prompt the thyroid to behave in ways that it wouldn't under normal circumstances. One in 8 women will have an issue with her thyroid in her lifetime, so the more information you can gather about how thyroid disorders could impact you, the more action you can take.
If you have a family history of thyroid issues, like thyroid disease or postpartum thyroid problems - we'll get to those later - you should make your doctor aware of that. (Preparing to become a parent is a great time to ask your relatives about their medical histories, especially the women in your life, not just about medical problems, but about their experiences of pregnancy and birth.)
During pregnancy, the risk of developing hypothyroidism - that's when the thyroid doesn't produce enough thyroid hormone - increases, especially if you're older when you get pregnant. The thing about hypothyroidism during pregnancy is that it looks like a lot of the symptoms your run into when you're pregnant, like weight gain, constipation, carpal tunnel syndrome, and muscle cramps. Because these are associated with pregnancy, hypothyroidism often gets overlooked, and if your hypothyroidism is super mild, you might not have any symptoms at all. Make sure you're keeping up with your prenatal visits, and if you feel like something is off, bring it up with your doctor. Even if it turns out to be nothing, it's still better to have it checked out. If you're not producing enough thyroid hormones, that means your baby isn't getting enough, and for the first few months of life, they depend on you for that. A lack of thyroid hormones can result in neurological problems, and untreated, hypothyroidism can causes problems during pregnancy, such as preeclampsia.
Testing for hypothyroidism during pregnancy is something doctors are conflicted about, so not everyone will test for it, unless they believe there's a reason to think you have it. If you are diagnosed with hypothyroidism, you'll be given thyroid hormone replacement therapy (safe to take during pregnancy) and monitored throughout, and once your baby is born, their thyroid levels will be assessed. Your thyroid level may or may not return to normal, depending on the cause of your hypothyroidism.
Sources
https://avivaromm.com/thyroid-in-pregnancy/
https://www.endocrineweb.com/conditions/thyroid/pregnancy-your-thyroid
https://www.hormone.org/diseases-and-conditions/thyroid/hypothyroidism
https://www.endocrineweb.com/conditions/thyroid/hypothyroidism-too-little-thyroid-hormone