A Guide to Normal Thyroid Levels in Pregnancy

pregnancy Feb 21, 2025

Young pregnant woman looking out the window smiling

When you're pregnant, your body goes through incredible changes to support both you and your baby. One of the most important players in this process is your thyroid gland. This butterfly-shaped gland in your neck produces hormones critical for your baby’s development and your overall well-being. Ensuring normal thyroid levels in pregnancy is essential for a healthy pregnancy journey.

Let’s break down what you need to know about thyroid health during pregnancy, how it’s tested, and why it matters.

What Are Normal Thyroid Levels in Pregnancy?

Thyroid-stimulating hormone (TSH) is the main marker used to evaluate thyroid function. During pregnancy, the normal TSH levels shift slightly depending on the trimester:

First trimester: 0.1-2.5 mIU/L
Second trimester: 0.2-3 mIU/L
Third trimester: 0.3-3 mIU/L

These trimester-specific ranges reflect the natural changes in your body, such as the influence of pregnancy hormones and the increased demand for thyroid hormones.

Why Does Thyroid Function Change During Pregnancy?

During pregnancy, your thyroid undergoes significant changes to support your and your baby's increased metabolic demands. The thyroid gland can enlarge by about 10% in populations with adequate iodine intake and by 20–40% in iodine-deficient populations. To meet these new demands, thyroid hormone levels adjust in several ways.

Increased Thyroid Hormone Production

Thyroxine (T4) and triiodothyronine (T3) production rises by about 50% to maintain metabolic balance. This increase is crucial for the baby’s early development, particularly in the brain and nervous system, as the baby relies on maternal thyroid hormones during the first trimester.

Iodine Requirements and Renal Clearance

Pregnancy also raises your body’s need for iodine by 50%. This is due to increased renal clearance (more iodine excreted by the kidneys) and the baby’s growing thyroid demands. Without sufficient iodine, thyroid function can become impaired, leading to potential complications.

The Role of hCG in Early Pregnancy

Human chorionic gonadotropin (hCG), a hormone produced during pregnancy, acts like thyroid-stimulating hormone (TSH) by promoting the secretion of thyroid hormones. This can temporarily suppress TSH levels, especially early in pregnancy when hCG peaks.

Elevated Thyroid Binding Proteins

Estrogen plays a role by increasing the production of thyroxine-binding globulin (TBG), a protein that transports thyroid hormones in the blood. This leads to a higher proportion of bound thyroid hormone and a decrease in the “free” thyroid hormone level, making free thyroid hormone less readily available to tissues. These increased TBG levels often require adjusting thyroid hormone replacement medication dosage for individuals with hypothyroidism early in pregnancy.

Proper iodine intake and thyroid monitoring are critical in the early stages of pregnancy and throughout.

Why Is Thyroid Health Crucial During Pregnancy?

Thyroid hormones are vital for your baby’s growth and development, particularly their brain and nervous system in the first trimester. Dysregulated thyroid hormone can lead to serious complications, including:

Maternal Risks

Miscarriage or early pregnancy loss
Gestational hypertension
Preeclampsia
Postpartum hemorrhage

Neonatal Risks

Preterm birth
Low birth weight
Developmental delays
Congenital malformations

How Is Thyroid Function Tested?

Thyroid function is typically assessed through a blood test that measures thyroid-stimulating hormone (TSH) levels. Additional tests may include free T3, free T4, and thyroid antibodies to provide a more comprehensive picture of thyroid health. Thyroid testing is particularly crucial for individuals who are at a higher risk of thyroid disease.

Who Is at Risk of Thyroid Disease?

Women with the following risk factors should consider thyroid testing, especially during pregnancy:

Personal History: Hypothyroidism, hyperthyroidism, thyroid symptoms, goiter, or thyroid antibody positivity
Medical History: Previous head/neck radiation or thyroid surgery
Age: Greater than 30 years
Autoimmune Conditions: Conditions such as type 1 diabetes or polycystic ovary syndrome (PCOS)
Reproductive History: Pregnancy loss, preterm birth, infertility, or having two or more prior pregnancies
Family History: Autoimmune thyroid disease or thyroid dysfunction
Obesity: Morbid obesity with a BMI of 40 or higher
Medication Exposure: Use of medications such as amiodarone or lithium, or recent exposure to iodinated contrast agents
Geographic Factors: Living in regions with moderate to severe iodine deficiency

Challenges in Thyroid Testing During Pregnancy

While guidelines recommend specific TSH ranges for pregnancy, testing practices can vary widely. For example, a study in Sweden revealed gaps between recommended testing and actual practice. One reason may be that guidelines are developed by endocrinologists, while obstetricians often manage the care of pregnant women.

It’s also important to note that universal thyroid testing in pregnancy is still debated. Some experts advocate for testing all pregnant women, given the commonality of thyroid issues and the simplicity of testing, while others suggest focusing on high-risk individuals.

Managing Thyroid Levels Holistically

As naturopathic doctors, we believe in treating thyroid imbalances with a holistic approach. Proper treatment in pregnancy is essential and we often use an integrative approach for our patients - a combination of conventional and natural medicine. While conventional treatment may rely on synthetic thyroid hormone (levothyroxine) alone, it doesn’t address underlying causes like nutrient deficiencies, stress, or lifestyle factors.

Here’s how we approach thyroid health during pregnancy:

Dietary Support
- Incorporate iodine-rich foods like fish, dairy, and seaweed.
- Include selenium (found in Brazil nuts) and zinc (found in oysters and poultry) to support thyroid function.
Targeted Supplementation
- Ensure adequate levels of vitamins and minerals like magnesium, vitamin D, and iron.
- Supplement only when clinically necessary to avoid over-supplementation.
Lifestyle Adjustments
- Manage stress through mindfulness, meditation, and adequate sleep.
- Regular, gentle exercise to boost overall well-being.
Collaborative Care
- Combine conventional and naturopathic approaches for individualized care.
Universal Testing and Medication as Needed
- Test everyone and advocate for thyroid hormone prescribing when needed. We are often the doctors managing these hormones for our patients.

Takeaway

Ensuring normal thyroid levels in pregnancy is vital for both you and your baby’s health. Whether you’re planning a pregnancy, currently pregnant, or postpartum, understanding and managing your thyroid function can help prevent complications and set the foundation for a healthy future.

If you suspect thyroid issues or want to optimize your thyroid health, schedule a free discovery call. Together, we’ll create a plan to support your journey through pregnancy and beyond.