Thyroid problems in pregnancy: what every mother should know

Thyroid problems are among the most common hormone issues in pregnancy, and in India they are common in young women in general. The good news is that they are easy to test for and, when found early, they are very manageable. This page explains why the thyroid matters so much in pregnancy, how the tests are read, and what usually happens next. It does not replace the advice of your obstetrician or endocrinologist.

Why does the thyroid matter so much in pregnancy?

Thyroid hormone is essential for a baby’s brain and nerve development. In the first three months, the baby’s own thyroid is not yet working properly, so it depends completely on the mother’s thyroid hormone. Later in pregnancy the baby’s thyroid takes over, but it still needs iodine from the mother.

To supply both of you, a pregnant woman’s thyroid has to work much harder. Her body also makes more of the proteins that carry thyroid hormone, and the pregnancy hormone hCG stimulates the thyroid. A healthy thyroid copes easily. A thyroid that is already weak, for example from Hashimoto’s, may not.

Untreated thyroid problems in pregnancy have been linked with miscarriage, high blood pressure, preterm birth and effects on the baby’s development. Getting it right early reduces these risks.

When should the thyroid be tested?

Ideally before pregnancy, if you are planning one, and otherwise as early as possible after a positive test, at the first antenatal visit. International guidelines recommend testing women at higher risk, such as those with a past thyroid problem, a goitre, positive thyroid antibodies, type 1 diabetes, a family history, previous miscarriage or infertility. Because thyroid problems are so common in India, many Indian obstetricians and endocrinologists check TSH in every pregnant woman.

The basic test is TSH, often with FT4. Your doctor may add anti-TPO antibodies. See the thyroid function test explained.

Why are the “normal ranges” different in pregnancy?

Pregnancy changes the numbers even in a healthy woman:

• In the first trimester, hCG gently stimulates the thyroid, so TSH naturally falls, sometimes below the usual lab range.
• Total T4 and T3 rise because carrier proteins increase, so total values are read differently.
• In the second and third trimesters, TSH drifts back up.

Because of this, doctors use trimester-specific ranges, ideally from the local population and the same lab method. When local ranges are not available, guidelines suggest using adjusted cut-offs for pregnancy. A TSH that looks normal on the printed lab range may be judged high in early pregnancy, and the reverse is also true. This is why a pregnant woman’s report should be read by her doctor, not just against the range printed on the page.

What if my thyroid is underactive?

If you are already on thyroid tablets: your need for thyroid hormone usually rises early in pregnancy. Tell your doctor as soon as your pregnancy test is positive. Many doctors review the strength immediately and then check TSH every few weeks in the first half of pregnancy. Please do not change the strength yourself.

If it is newly found: overt hypothyroidism (high TSH, low T4) is treated with levothyroxine, which is safe in pregnancy and while breastfeeding. Mild, or subclinical, hypothyroidism is treated more readily in pregnancy than outside it, especially when anti-TPO antibodies are positive. Read high TSH with normal T4.

Levothyroxine works best when taken the same way every day, apart from iron and calcium tablets, which most pregnant women in India are also given. Read how to take levothyroxine the right way.

What if my thyroid is overactive?

A low TSH in early pregnancy is often caused by hCG, especially with severe vomiting. This is called gestational transient thyrotoxicosis. It usually settles by the middle of pregnancy and rarely needs anti-thyroid medicines. Read low TSH with normal T4.

Graves’ disease is different and needs specialist care. The choice of anti-thyroid medicine changes during pregnancy to protect the baby, and the lowest effective treatment is used. Women who have had Graves’ disease in the past, even if treated with radioactive iodine or surgery, may still carry antibodies (TRAb) that can cross to the baby, so these are usually checked. Radioactive iodine is never given in pregnancy.

If you take carbimazole or methimazole and your pregnancy test is positive, contact your doctor the same day. Do not change or stop it on your own.

What happens after delivery?

• Women on levothyroxine for an underactive thyroid that started in pregnancy may be able to go back to their earlier treatment or stop, as their doctor advises. TSH is usually checked about six weeks after delivery.
• Postpartum thyroiditis can occur in the first year after birth, especially in women with positive anti-TPO antibodies. It may cause a short overactive phase (palpitations, anxiety), then an underactive phase (tiredness, low mood). It is easily mistaken for the normal exhaustion of new motherhood.
• Babies are screened for congenital hypothyroidism after birth.

What about iodine?

Pregnant and breastfeeding women need more iodine. In India, iodised salt covers most of this. Avoid kelp or seaweed supplements unless your doctor advises them, as too much iodine can also upset the thyroid.

What should you ask your doctor?

• Should my thyroid be checked before I try to conceive?
• What TSH range are you aiming for in each trimester?
• How often will my thyroid be tested during pregnancy?
• How should I space my thyroid tablet from iron and calcium?

Pregnancy also brings other hormone checks, such as for sugar. Read what causes gestational diabetes. If you have a thyroid condition and are planning a pregnancy, book a consultation with an endocrinologist.