Thyroid diseases are among the most common medical problems I see in my pregnant patients. In around 2% of pregnancies the mother is hypothyroid (she has an underactive thyroid gland). Hyperthyroidism (an overactive thyroid gland) is less common, affecting 1-in-500 pregnancies.
What is the thyroid?
The thyroid is a small gland located in your neck, a little below the Adam’s apple. It makes hormones that play a key role in controlling your metabolism. Thyroid hormones are also very important for growth and development, including the development of your unborn baby’s brain.
Hypothyroidism (an underactive thyroid)
An underactive thyroid does not make enough thyroid hormone to keep up with your needs. Hashimoto’s thyroiditis, an auto-immune condition, is the most common cause of underactive thyroid in Australia. In other parts of the world, the most common cause is iodine deficiency. Other causes include medication, surgery and infections.
The good news is that an underactive thyroid is easy to treat. We can replace your thyroid hormone with a tablet. If you are taking the right dose of hormone from pre-pregnancy to birth, your thyroid disease shouldn’t cause any problems.
On the other hand, untreated hypothyroidism can lead to infertility, miscarriage, fetal growth restriction and other pregnancy problems. It is also associated with lower IQ or developmental delay in children.
It’s a good idea for women with underactive thyroid to have a blood test before falling pregnant. This way your doctor can make sure you are on the right dose of hormone before you conceive. Once you are pregnant, you will need extra monitoring of your thyroid tests and medication dose. Interpreting thyroid tests during pregnancy is not always straightforward, so it’s important to see an obstetrician with experience of thyroid disease.
Very rarely (1-in-180000), a baby can develop an underactive thyroid while still in the womb. All babies are checked for hypothyroidism on the newborn heel-prick test.
Sub-clinical hypothyroidism (SCH)
SCH is a mild form of thyroid dysfunction. Lots of studies have looked at SCH in pregnant women and the results have been conflicting. If you have SCH, you will need to discuss your own circumstances and need for treatment with an experienced high-risk obstetrician.
Hyperthyroidism (an over-active thyroid)
An overactive thyroid makes more thyroid hormone than you need. Graves disease, an auto-immune condition, is the most common cause of hyperthyroidism. Other causes include nodules in the gland, inflammation (called thyroiditis) or medication.
Most women with an overactive thyroid are diagnosed and started on treatment before pregnancy. Some women are diagnosed for the first time during pregnancy, usually in the first 20 weeks.
Several medications are used to treat hyperthyroidism. Some of these drugs affect the fetus. It’s very important to see a Maternal-Fetal Medicine specialist, who will work with your thyroid specialist to find a treatment option that is safe for both you and your baby.
During pregnancy, women with overactive thyroid often need to adjust their medication and increase the frequency of their blood tests. Flare-ups are particularly common in the first trimester and immediately after birth. It’s really important to keep thyroid disease well controlled throughout pregnancy. Both mother and baby usually do well if the overactive thyroid is well-controlled. However, poorly controlled hyperthyroidism increases the chance of complications like miscarriage, fetal growth restriction and premature birth.
In a small number of cases, mothers have thyroid antibodies that cross the placenta and cause an overactive thyroid in the fetus. This is a very serious complication. Mothers with this condition need to see a Maternal-Fetal Medicine specialist for the rest of the pregnancy. The baby will also need to see a neonatologist in the first days after birth.
Around 5-10% of women will develop inflammation in their thyroid gland in the first year after giving birth. The symptoms are usually very subtle, so the condition is often missed. The most common symptoms are fatigue, insomnia, changes in weight and mood changes – all extremely common even in healthy new mums.
Post-partum thyroiditis can cause over- or under-active thyroid, and can switch from one to the other in the same patient. Most women recover on their own, without needing any treatment. However, it can come back after future pregnancies. A number of women will recover initially, but go on to develop permanent thyroid problems in later life, so long-term follow-up is important.