Thyroid and Pregnancy: Hypothyroidism, Screening and Follow-up

You have taken a thyroid tablet every morning for years, and your pregnancy test has just come back positive. Or a TSH test was included in your first set of bloods, without anyone really explaining why. Thyroid and pregnancy are closely linked: this small gland in your neck makes hormones that your baby needs very early on, even before its own thyroid starts working.

Most thyroid problems are very well managed during pregnancy, as long as they are spotted and followed up. Here is what you need to know, whether or not you are already on treatment.

Why does the thyroid matter so much in pregnancy?

Because the baby first borrows its mother's hormones. A document from Grenoble University Hospital points out that the fetal thyroid starts to form at around 10 to 12 weeks, and that during the first trimester the baby depends on the mother's thyroid hormones. Throughout pregnancy, it also depends on the iodine provided by its mother's diet. These hormones are essential for the development of its brain and nervous system.

Pregnancy also increases the mother's own needs. That is why a thyroid that was enough before can become insufficient once you are pregnant, and why treatment often needs adjusting.

Hypothyroidism in pregnancy: what are the risks?

Hypothyroidism means an underactive thyroid that does not make enough hormones. Its most common cause is Hashimoto's thyroiditis, an autoimmune disease in which the body attacks its own thyroid. It mainly affects women, often between the ages of 30 and 50, according to the UK's NHS. A lack of iodine in the diet can also cause it.

Left untreated, it increases the risk of, according to the NHS:

• miscarriage;
• pre-eclampsia, a form of high blood pressure in pregnancy (see our article on pre-eclampsia);
• premature birth;
• birth defects.

The NHS stresses that treatment usually prevents these complications. That is what follow-up is all about.

The symptoms are often subtle and resemble those of pregnancy: extreme tiredness, feeling the cold, weight gain, constipation, dry skin and hair, difficulty concentrating, low mood. Intense tiredness can also come from anaemia during pregnancy: only a blood test can tell them apart.

Who should have their thyroid checked?

Should every pregnant woman have a TSH test? Specialists disagree. A 2015 Cochrane review covering more than 26,000 women showed that routine screening leads to more women being treated, but with no clearly demonstrated benefit on pre-eclampsia, premature birth or the children's development at age three. Many teams therefore use targeted screening.

The Grenoble University Hospital document lists, among others, these situations as justifying a test:

• a personal or family history of thyroid disease;
• an autoimmune disease, such as type 1 diabetes;
• infertility or repeated miscarriages;
• suggestive symptoms: extreme tiredness, dry skin, hair loss, goitre (a swollen neck), swelling, excessive weight gain.

The test is a simple blood test that measures TSH, the hormone that controls the thyroid, sometimes alongside free T4 and an antibody test. In Morocco, it is prescribed by your gynaecologist or doctor and can be done at any laboratory. Ideally, have it done before you conceive: it is one of the checks at your pre-conception appointment.

Already on thyroid treatment: what to do when you're pregnant

Tell your doctor as soon as your test is positive, without waiting for the first scan. The NHS says the dose of your hypothyroidism treatment will probably need to change during pregnancy, and the Grenoble document specifies that the adjustment is made as soon as the pregnancy is confirmed, because your needs rise early.

In practice:

1. Do not change anything yourself. No increase “just in case”, no stopping. The dose is set by the doctor based on your results.
2. Have the blood tests you are asked for. TSH is checked regularly during pregnancy, often every month at first, then depending on how stable it is. The target values are specific to pregnancy: your doctor interprets them.
3. Mention all your supplements. Iron, calcium, prenatal vitamins: tell your doctor or pharmacist what you are taking, and they will tell you how to time them with your treatment.
4. After the birth, the dose is usually brought back to your pre-pregnancy dose, always on medical advice, with a check a few weeks later.

What about hyperthyroidism in pregnancy?

Hyperthyroidism is the opposite: too much hormone. It shows up as nervousness, palpitations, trembling, sensitivity to heat, sleep problems and weight loss. About 4 cases in 5 are due to Graves' disease, an autoimmune disease, according to the NHS. If poorly controlled, it too carries a risk of pre-eclampsia, premature birth and miscarriage.

It needs specialist follow-up, because some treatments are not suitable in pregnancy and the mother's antibodies can affect the baby's thyroid. If you are being treated for hyperthyroidism and would like a child, discuss it before you conceive.

One particular case is common in the first trimester: in women who vomit a lot, the pregnancy hormone hCG can stimulate the thyroid and cause temporary hyperthyroidism, which improves on its own around 16 to 20 weeks of amenorrhoea (counted from your last period). If your pregnancy sickness is very severe, your doctor may look for it.

Iodine: iodised salt, one simple habit

The thyroid makes its hormones from iodine, and your needs rise during pregnancy and breastfeeding. Morocco has had a significant deficiency: a national survey in 1993 found goitre in 22% of children aged 6 to 12. In response, a decree published in the Official Bulletin in December 1995 made it compulsory to iodise salt intended for food.

What this means for you:

• use iodised salt for cooking and at the table, checking for the mention on the packet;
• without overdoing it: the aim is iodised salt, not more salt, especially if your blood pressure needs watching;
• do not take iodine or seaweed supplements on your own initiative: if a supplement is useful, your doctor prescribes it and sets the dose.

After the birth: postpartum thyroiditis

In the year after the birth, the thyroid can become temporarily inflamed. According to the American Thyroid Association, this postpartum thyroiditis affects about 5 to 10% of women in the United States. It often unfolds in two stages:

• a hyperthyroid phase, 1 to 4 months after the birth: anxiety, insomnia, palpitations, irritability, weight loss;
• then a hypothyroid phase, usually 4 to 8 months after: tiredness, weight gain, constipation, dry skin, low mood.

These signs are easily put down to lack of sleep or to postnatal depression. Most women get back to normal thyroid function within 12 to 18 months, but about 20% of those who go through the hypothyroid phase stay that way, and thyroiditis comes back in about 20% of women in their next pregnancy. The risk is higher with thyroid antibodies, type 1 diabetes or a history of thyroid problems.