Thyroid and pregnancy

For many women in India, the first thyroid test of their life comes at the first antenatal visit. A result outside the range can be a worry when you are already carrying a lot of new information. In most cases, thyroid problems in pregnancy are easy to manage once they are found, and finding them is the whole point of testing.

Why pregnancy changes the thyroid

In early pregnancy, the baby relies entirely on the mother's thyroid hormone, because its own thyroid only starts working properly at around 12 weeks and remains immature for a long time. At the same time, the pregnancy hormone hCG mildly stimulates the mother's thyroid, and other hormone changes raise the amount of thyroid hormone the body needs. Together, these push TSH down in the first trimester and increase the demand on the thyroid throughout pregnancy.

What is a normal TSH level in pregnancy?

The 2017 American Thyroid Association pregnancy guideline recommends that labs use reference ranges built from healthy pregnant women in their own population, for each trimester. Where those are not available, it suggests an upper limit of about 4.0 mIU/L for TSH in early pregnancy. Many Indian labs print trimester-specific ranges on the report; use those first.

For a general guide to TSH, free T4 and antibodies, see thyroid tests explained.

Who is tested?

International guidelines recommend testing women at higher risk, including those with a personal or family history of thyroid disease, thyroid antibodies, a goitre, type 1 diabetes or other autoimmune conditions, previous miscarriage or preterm birth, infertility, or who are over 30. Because thyroid problems are so common in Indian women, many doctors in India check TSH in every pregnant woman at the first visit. Testing as early as possible, ideally before 9 weeks or when planning pregnancy, gives the most useful information.

Hypothyroidism in pregnancy

Untreated overt hypothyroidism is linked to miscarriage, high blood pressure in pregnancy, preterm birth and effects on the baby's development, so it is always treated with levothyroxine. Treated well, most women have healthy pregnancies and babies.

• Already on thyroxine? Your dose usually needs to go up as soon as pregnancy is confirmed. Contact your doctor straight away when you have a positive test; they will tell you how to adjust it and when to recheck. Do not change the dose yourself.
• Blood tests are usually repeated about every four weeks in the first half of pregnancy and at least once around 26 to 32 weeks.
• Subclinical hypothyroidism: whether to treat depends on the TSH level and on anti-TPO antibodies. Your doctor will explain the reasoning for your situation.
• After delivery, most women go back to their pre-pregnancy dose, with a check about six weeks later.

More on the condition itself is in our hypothyroidism guide.

Hyperthyroidism in pregnancy

A low TSH in early pregnancy is often caused by hCG rather than true thyroid disease, especially in women with severe vomiting (hyperemesis). This usually settles by mid-pregnancy without antithyroid treatment. Graves' disease is the main cause of true hyperthyroidism in pregnancy. It needs specialist care, because the choice of antithyroid medicine differs between the first trimester and later pregnancy, and because the antibodies can cross the placenta and affect the baby's thyroid. Radioactive iodine is never used in pregnancy. See our hyperthyroidism guide for background.

Iodine in pregnancy

The baby's brain development needs iodine, and the body's need rises in pregnancy and breastfeeding. In India, using iodised salt at home is the main source; rock salt, sea salt and some “natural” salts are often not iodised. Many prenatal supplements also contain iodine. Ask your doctor whether yours does, and avoid seaweed or kelp supplements, which can contain far too much.

Planning a pregnancy

• If you have hypothyroidism, ask your doctor to check your TSH before you try to conceive, so it is on target from the start.
• If you have hyperthyroidism, discuss treatment choices before pregnancy; some options are better completed beforehand.
• After radioactive iodine, pregnancy needs to be avoided for a period; ask your doctor how long.
• Thyroid problems can make periods irregular and conception harder; treating them helps.

After the baby: postpartum thyroiditis

In the year after delivery, some women develop inflammation of the thyroid. It often causes a short overactive phase (palpitations, anxiety, weight loss) followed by an underactive phase (tiredness, low mood, hair fall), before returning to normal. Symptoms are easily put down to a new baby and broken sleep. Women with thyroid antibodies are at higher risk, and some go on to permanent hypothyroidism, so ask for a TSH test if you feel unwell, especially if you have low mood after delivery.

Questions to ask your doctor

• Which TSH range does my lab use for this trimester?
• Should I have an anti-TPO antibody test?
• If I take thyroxine, how should my dose change now, and when is my next test?
• Does my prenatal supplement contain iodine?
• When should my thyroid be checked after delivery?

Frequently asked questions

Sources

1. Alexander EK, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid 2017;27(3):315–389.
2. Unnikrishnan AG, et al. Prevalence of hypothyroidism in adults: an epidemiological study in eight cities of India. Indian J Endocrinol Metab 2013;17(4):647–652.

This article is general information for adults in India and is not a diagnosis or a treatment plan. Always discuss results and any changes with your own doctor. Read our editorial policy.