Thyroid problems are surprisingly common during pregnancy, affecting approximately 1 in 25 pregnancies in various forms. Many expecting mothers dismiss symptoms like extreme fatigue, brain fog, and unexplained weight gain as normal pregnancy complaints, not realizing their thyroid may be working overtime to support both their own body and their baby's developing brain and nervous system.

Why Does the Thyroid Change During Pregnancy?

Your thyroid undergoes dramatic shifts the moment you become pregnant. The pregnancy hormone human chorionic gonadotropin (hCG), which is detected by pregnancy tests, is structurally similar to thyroid-stimulating hormone (TSH). This similarity causes the thyroid to produce more hormone in early pregnancy, a natural response designed to ensure adequate thyroid hormone for the developing fetus.

During the first 12 weeks of pregnancy, your baby is entirely dependent on you for thyroid hormones because their own thyroid gland doesn't begin functioning until around the end of the first trimester. This means your thyroid must increase production by 40% to 50% to meet the needs of two people. Additionally, pregnancy increases how quickly your kidneys clear iodine from your body, and your baby also requires a share of your iodine supply. In the UK, where iodine deficiency can sometimes be an issue, this increased demand can strain the thyroid and potentially lead to an underactive state if levels aren't maintained through diet or supplementation.

How Common Are Thyroid Issues in Pregnancy?

Research shows that around 2% to 3% of pregnancies involve hypothyroidism, an underactive thyroid, while hyperthyroidism, an overactive thyroid, occurs in about 1 in every 1,000 pregnancies. However, subclinical issues, where hormone levels are slightly out of the optimal range but not yet causing severe disease, are even more frequent. Some studies indicate that when testing all pregnant women, up to 12% may show some form of abnormal thyroid function in early pregnancy.

For many women, a thyroid issue may have existed before pregnancy but was never diagnosed because symptoms were mild. The physiological stress of pregnancy can bring these underlying tendencies to the surface. For others, the condition is purely gestational, triggered by the unique hormonal environment of pregnancy itself.

What Are the Signs of an Underactive Thyroid During Pregnancy?

Hypothyroidism, or an underactive thyroid, is most commonly caused by Hashimoto's disease, an autoimmune condition where the immune system mistakenly produces antibodies that attack thyroid tissue. The challenge with identifying hypothyroidism during pregnancy is that many of its symptoms are classic pregnancy complaints, making them easy to overlook.

• Extreme Fatigue: Feeling unusually run down and exhausted beyond typical pregnancy tiredness.
• Temperature Sensitivity: Feeling unusually cold or sensitive to low temperatures when others feel comfortable.
• Digestive Issues: Persistent constipation that doesn't respond to typical remedies.
• Skin and Hair Changes: Dry skin and brittle hair or nails that worsen during pregnancy.
• Cognitive Difficulties: Brain fog or difficulty concentrating that interferes with daily tasks.
• Unexplained Weight Gain: Gaining weight above what is expected for your stage of pregnancy.

If left untreated, a significantly underactive thyroid can increase the risk of serious complications such as preeclampsia, a dangerous rise in blood pressure, anemia, and low birth weight. Most importantly, because thyroid hormone is vital for brain development, untreated hypothyroidism can affect your baby's cognitive growth.

How to Manage Thyroid Health During Pregnancy

• Start with Your GP: Have a conversation with your general practitioner to rule out immediate concerns and discuss your symptoms in the context of your pregnancy timeline.
• Track Your Symptoms: Keep a record of how you're feeling, including energy levels, temperature sensitivity, mood changes, and any digestive issues, to help your healthcare team identify patterns.
• Consider Structured Blood Testing: If you're still concerned after speaking with your GP, use structured blood testing to get a detailed snapshot of your thyroid function that helps you have a more productive conversation with your medical professional.
• Monitor Iodine Intake: Ensure your diet includes adequate iodine through foods like dairy products, eggs, and seafood, or discuss supplementation with your healthcare provider if you're at risk of deficiency.
• Adjust Medication if Needed: If you were already taking levothyroxine, a synthetic thyroid hormone, before becoming pregnant, your GP will likely need to increase your dose as soon as your pregnancy is confirmed to account for the extra demand.

The good news is that hypothyroidism is highly manageable. Most women are treated with levothyroxine, a synthetic version of the hormone T4, which is safe and effective during pregnancy.

What About an Overactive Thyroid During Pregnancy?

Hyperthyroidism, when the thyroid produces an excess of hormones, is less common than an underactive thyroid but requires careful monitoring. The most frequent cause of persistent hyperthyroidism in pregnancy is Graves' disease, another autoimmune condition where the immune system creates antibodies that stimulate the thyroid to overproduce hormones. Interestingly, for some women with Graves' disease, symptoms may actually improve in the second and third trimesters as the immune system naturally shifts during pregnancy.

If you experience sudden or severe symptoms such as a very rapid or irregular heartbeat, difficulty breathing, severe tremors, or swelling of the lips, face, or throat, seek urgent medical help immediately by calling 999 or attending your local accident and emergency department.

What Happens to Your Thyroid After Pregnancy?

Postpartum thyroiditis is another consideration for expecting mothers. This condition can develop after birth when the thyroid becomes inflamed, potentially causing temporary hyperthyroidism followed by hypothyroidism. Understanding this possibility helps you recognize symptoms in the months following delivery and seek appropriate care if needed.

The key takeaway is that thyroid health during pregnancy is not something to ignore. Because symptoms of thyroid dysfunction so closely mirror standard pregnancy discomforts, many issues go unnoticed. By staying aware of how you're feeling, maintaining open communication with your healthcare team, and pursuing appropriate testing when needed, you can help ensure both you and your baby receive the thyroid hormone support necessary for healthy development.