Pregnancy has a significant effect on the thyroid function and it’s fairly common for a pregnant woman to develop thyroid disease. Thyroid disease can impact many aspects of the pregnancy and post-partum health as well as the health of the baby. Your thyroid gland is important during pregnancy as it regulates the production of hormones called T3 and T4, both of which play a critical role in the development of the baby’s brain and nervous system.

During the first trimester, the foetus depends on the mother’s supply of thyroid hormone, which is delivered through the placenta. In order to meet the baby’s need, the mother’s thyroid production will typically go into overdrive, resulting in an enlargement of the gland. This usually will not complicate the pregnancy in any way.

Beyond the physical enlargement of the gland itself, there are changes in hormone production which can be monitored through blood tests. Chief among them is the “TSH” test. Because normal thyroid function is different during pregnancy, TSH values will change as the mother progresses from first to the third trimester.

If for any reason the thyroid gland cannot keep up during pregnancy, the value will drop, indicating hypothyroidism state. In such case, thyroid hormone replacement medication will be prescribed to replace the missing thyroid hormone. The pregnant woman’s thyroid profile will then be monitored to adjust the treatment as needed. If thyroid disease runs in your family or you are a known patient with the thyroid problem, it is important to inform your doctor so that you can be properly monitored and treated.

According to the American Thyroid Association (ATA) guidelines, these increases in the dose of thyroid hormone replacement should begin as early as 4-6 weeks and continue through 16-20 weeks after which it will typically plateau until delivery. Thyroid levels need to be tested every four weeks during the first half of pregnancy and thereafter once or twice between 26-32 weeks till delivery. Post-delivery medication doses will usually drop down to pre-pregnancy values and will need to be monitored for about six weeks after the baby has been delivered.

One important thing to remember is that if you are hypothyroid and if you are planning to conceive, you need to talk with your doctor about adjusting your dosage of thyroid medication in order to optimise fertility. The goal is to maintain your TSH level below 2.5 mu/L.

New evidence-based recommendations from the ATA 2017 provide guidelines to diagnose and manage thyroid disease during pregnancy and the postpartum period. At least 2% – 3% of healthy non-pregnant women of childbearing age have hypothyroidism. The prevalence may be higher in areas of iodine deficiencies.

In the 2011 ATA guidelines, the upper reference limit for serum TSH level during pregnancy was defined as 2.5 mu/L in the first trimester and 3.0 mu/L in the second and third trimester. These cut-offs were based on published reference ranges obtained from a cohort of 5500 subjects. Since that publication, additional larger data from over 60,000 subjects has become available now. The new analysis shows that the TSH upper referral limit can vary significantly depending on the geography and ethnicity of the population.

The presence of TPO antibody (thyroid peroxidase is an enzyme in the thyroid gland which helps in the production of the thyroid hormone) suggests that the thyroid disease is being caused by an autoimmune disorder.

New recommendations – ATA (2017) that are worth highlighting for clinicians:

• The normal upper limit for thyroid function in pregnancy is revised to 4.0 (TSH level) from 2.5 in 2011. Initial studies of pregnant women in USA and Europe led to the earlier recommendation for a TSH upper reference limit of 2.5 mu/L in the first trimester, then 3.0 mu/L in the second and third trimester. However, recent studies have looked at women in Asia, India and the Netherlands finding only a modest reduction in upper reference limit of TSH. Looking at the evidence TSH value has been shifted to set 4.0 mu/L (upper hormone limit of TSH) in early pregnancy with a gradual return to non-pregnant levels also.
• If the presence of TPO antibody has been detected, treatment with levothyroxine (a manufactured form of the thyroid hormone which is used to treat thyroid hormone deficiency) is strongly recommended. The treatment should be fine-tuned to the specific needs of the patient.
• Ideally, women with hypothyroidism should be checked before they conceive to ensure their thyroid gland is functioning normally. And they should be tested soon after they become pregnant to monitor their TSH and FT4 levels and increase their thyroid dose as needed.
• Another important recommendation is to make sure that your prenatal vitamins include iodine – an essential nutrition for thyroid function during pregnancy.

Breastfeeding with thyroid disease:

-Many new mothers choose to breastfeed if they are being treated for hypothyroidism.
-While breastfeeding you can safely continue to take thyroid hormone replacement at appropriate dose without any harm to your baby.
-The question of taking anti-thyroid drugs for hyperthyroidism while breastfeeding is a lot more controversial and you should explore pros and cons further before making any decisions.
-Recent studies have reported that screening of only women considered high risk would miss 30% of women with overt or subclinical hypothyroidism. So we advocate universal screening for thyroid problems.