OVERVIEW

What is hypothyroidism during pregnancy?

The thyroid is an endocrine organ that secretes thyroid hormones. If thyroid hormone synthesis and secretion decrease due to various reasons, it results in hypothyroidism, commonly referred to as underactive thyroid.

Whether hypothyroidism is present before pregnancy or discovered during pregnancy, it is called hypothyroidism during pregnancy.

How common is hypothyroidism during pregnancy?

Hypothyroidism is a common thyroid disorder in pregnant women, affecting 3% to 5% of pregnancies.

What are the types of hypothyroidism during pregnancy?

This condition is classified into three types based on the type of hypothyroidism: primary clinical hypothyroidism, primary subclinical hypothyroidism, and central hypothyroidism.

SYMPTOMS

What are the common manifestations of hypothyroidism during pregnancy?

Patients with mild conditions may show no symptoms.

Typical patients exhibit symptoms such as cold intolerance, fatigue, swelling sensation in hands and feet, drowsiness, memory loss, reduced sweating, joint pain, constipation, slow response, hoarseness, dry and rough skin, edema, etc.

What are the risks of hypothyroidism during pregnancy?

Untreated clinical hypothyroidism during pregnancy has many adverse effects on the mother, including spontaneous abortion, premature birth, preeclampsia, gestational hypertension, postpartum hemorrhage, etc. It can also lead to lower fetal weight or even stillbirth, affecting the fetus's intelligence and motor abilities.

Pregnant women with untreated subclinical hypothyroidism also face an increased risk of miscarriage.

CAUSES

What are the common causes of hypothyroidism during pregnancy?

The most common cause of hypothyroidism during pregnancy is Hashimoto's thyroiditis. Other causes include iodine deficiency, radioactive I¹³¹ therapy for hyperthyroidism, post-thyroidectomy, medications (such as antithyroid drugs like thiourea, lithium carbonate, phenylbutazone, etc.), and pituitary or hypothalamic disorders.

DIAGNOSIS

What tests are needed when hypothyroidism during pregnancy cannot be confirmed?

Many women with hypothyroidism during pregnancy have no noticeable symptoms and are incidentally diagnosed during routine prenatal thyroid function tests.

If symptoms of hypothyroidism mentioned earlier occur during pregnancy, hypothyroidism can also be diagnosed through thyroid function tests, thyroid autoantibody tests, and thyroid ultrasound.

• Thyroid function tests: Include thyroid-stimulating hormone (TSH), thyroxine (T4), etc., used to diagnose hypothyroidism and assess its severity.

• Thyroid autoantibodies: Include thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), etc., used to assist in identifying the cause and determining the presence of autoimmune thyroiditis, such as Hashimoto's thyroiditis.

• Thyroid ultrasound: Used to help diagnose the cause of hypothyroidism.

If central hypothyroidism is suspected, a hypothalamic-pituitary MRI should be performed after delivery for further diagnosis.

Should routine prenatal checkups include thyroid-related tests when hypothyroidism during pregnancy cannot be confirmed?

Most cases of hypothyroidism during pregnancy are silent, with no obvious discomfort. Waiting for symptoms to appear before testing may lead to missed diagnoses. Therefore, it is currently recommended to conduct thyroid-related screening in early pregnancy, at least for high-risk "target populations."

Target populations include:

• Pregnant women with symptoms such as fatigue, cold intolerance, or constipation suggestive of hypothyroidism;

• Pregnant women aged 30 or older;

• Pregnant women with a history of spontaneous abortion or preterm birth;

• Pregnant women with type 1 diabetes.

Prenatal care requirements vary across regions in China. If you belong to the above target population and thyroid tests are not arranged during your first prenatal visit, you may request additional testing.

Do thyroid function tests for pregnant women with hypothyroidism require fasting?

No.

What test results indicate primary clinical hypothyroidism?

Thyroid function tests showing elevated thyroid-stimulating hormone (TSH) levels and decreased free thyroxine (FT4) levels suggest primary clinical hypothyroidism.

What test results indicate primary subclinical hypothyroidism?

Thyroid function tests showing elevated TSH levels with normal FT4 levels suggest primary subclinical hypothyroidism.

What test results indicate central hypothyroidism?

Thyroid function tests showing decreased FT4 levels with low or suppressed TSH levels suggest central hypothyroidism. However, about 20% of patients may have normal or mildly elevated TSH levels.

What conditions are easily confused with hypothyroidism during pregnancy?

This condition may be confused with adrenal insufficiency, pituitary TSH-secreting adenomas, morbid obesity, and thyroid hormone resistance syndrome. The above-mentioned tests and clinical presentation can help differentiate these conditions.

TREATMENT

Which department should I visit for hypothyroidism during pregnancy?

Endocrinology is the first choice, but you can also go to obstetrics.

Is treatment necessary for hypothyroidism during pregnancy?

Yes, treatment is essential for all types of hypothyroidism during pregnancy.

How is hypothyroidism during pregnancy treated?

Oral levothyroxine is the primary medication for this condition.

What are the treatment goals for hypothyroidism during pregnancy?

For primary clinical or subclinical hypothyroidism during pregnancy, the goal is to control thyroid-stimulating hormone (TSH) levels:

• First trimester (1–3 months): TSH 0.1–2.5 mIU/L;

• Second trimester (4–6 months): TSH 0.2–3.0 mIU/L;

• Third trimester (after 6 months): TSH 0.3–3.0 mIU/L.

For central hypothyroidism during pregnancy, the goal is to maintain free thyroxine (FT4) and total thyroxine (TT4) levels within the normal range.

What precautions should be taken when using levothyroxine for hypothyroidism during pregnancy?

Levothyroxine should be taken once daily, preferably on an empty stomach in the morning, 0.5–1 hour before breakfast, as some foods may interfere with its absorption. If morning dosing is difficult, it can be taken at bedtime, at least 4 hours after dinner.

Levothyroxine should be taken at least 4 hours apart from other medications, as certain drugs (e.g., aluminum hydroxide, calcium carbonate, cholestyramine, sucralfate, ferrous sulfate, fiber supplements) may affect its absorption and metabolism.

Additionally, some medications (e.g., phenobarbital, phenytoin, carbamazepine, rifampin, isoniazid, lovastatin, amiodarone, sertraline, chloroquine) may accelerate levothyroxine clearance. If taking these, the levothyroxine dose may need adjustment.

Always inform your doctor about other medications to determine the correct dosage and timing.

If hypothyroidism was diagnosed before pregnancy, consult a doctor immediately after conception to adjust the dose.

How should levothyroxine dosage be adjusted before, during, and after pregnancy?

Women with preexisting hypothyroidism should have thyroid function and antibody tests immediately after pregnancy to adjust levothyroxine dosage.

If unable to see a doctor, increase the dose by 25%–30%. For example, if the pre-pregnancy dose was 50 µg/day, raise it to ~62.5 µg/day.

Postpartum, revert to the pre-pregnancy dose and recheck thyroid function and antibodies at 6 weeks to adjust as needed.

Does levothyroxine affect the fetus?

No, when properly adjusted to maintain thyroid function within target levels, levothyroxine does not harm the fetus.

Is follow-up monitoring required after treatment for hypothyroidism during pregnancy?

Yes.

For the first 5 months, check thyroid function every 4 weeks. If stable, extend to every 6 weeks.

Can levothyroxine be stopped after delivery for pregnancy-induced hypothyroidism?

For subclinical hypothyroidism first detected during pregnancy, levothyroxine can be discontinued postpartum, but thyroid function and antibodies should be rechecked at 6 weeks.

If thyroid autoantibodies are positive, annual thyroid function tests are recommended due to the risk of permanent hypothyroidism.

Can hypothyroidism during pregnancy be cured?

Hypothyroidism caused by subacute thyroiditis or painless thyroiditis often resolves spontaneously. Central hypothyroidism due to pituitary tumors may be cured postpartum with surgery. Other causes are rarely curable.

DIET & LIFESTYLE

Should pregnant women with hypothyroidism avoid iodized salt?

No.

Iodine excretion through the kidneys increases in pregnant women, and the fetal demand for iodine also rises, leading to a significantly higher iodine requirement compared to non-pregnant women. Urinary iodine tests can assess whether a pregnant woman is iodine-deficient.

Adequate iodine intake during pregnancy can be ensured by consuming iodized salt or taking nutritional supplements containing potassium iodide.

What dietary precautions should pregnant women with hypothyroidism take?

Pregnant women require more iodine than the general population, so iodine deficiency should be avoided. However, excessive iodine intake may worsen hypothyroidism or affect fetal growth.

The WHO recommends a daily iodine intake of at least 250 µg during pregnancy. For example, consuming 6 grams of salt (about one beer bottle cap) provides approximately 120–180 µg of iodine. The remaining 70–130 µg can be obtained from food and water.

Seafood is rich in iodine, but marine fish contain the least—e.g., 100 g of small yellow croaker provides 5.8 µg. In contrast, 100 g of seaweed, kelp, or shellfish contains much higher amounts. Thus, marine fish can be eaten regularly, while seaweed, kelp, and shellfish should be limited to 1–2 servings per week.

If iodine-containing supplements (typically ~150 µg per dose) are taken daily, additional seafood should be avoided.

Can pregnant women with hypothyroidism have a normal delivery?

Yes. With proper treatment, adverse effects can be minimized.

Is hypothyroidism during pregnancy hereditary?

Some causes, such as autoimmune thyroiditis (e.g., Hashimoto's thyroiditis) or genetic defects in thyroid hormone synthesis, can be inherited.

Hashimoto's thyroiditis is the most common cause of hypothyroidism in pregnancy. Will it always be passed to the child?

It may be inherited but is not guaranteed.

There is no specific prevention method yet. However, Hashimoto's thyroiditis is not difficult to treat and has minimal impact on daily life.

PREVENTION

Can hypothyroidism during pregnancy be prevented?

Some causes of hypothyroidism can be prevented, including avoiding iodine deficiency or excessive iodine intake, and refraining from using or overusing medications that inhibit thyroid synthesis.