During pregnancy, the thyroid gland increases in size, thyroid hormones are produced in greater amounts, and more iodine is needed for proper function of this important gland. While these changes take place naturally in healthy women, problems may occur in women who are predisposed to thyroid dysfunction because of various underlying factors.
It is well-established that overt hypothyroidism can cause poor pregnancy outcomes, such as increased risk of fetal loss. It also has been linked to lower IQ in offspring. Subclinical hypothyroidism has also been linked to potential problems. However, the topic of screening for and treating subclinical hypothyroidism has been widely debated in the field of endocrinology. In the past few years, a handful of studies has shed additional light on the subject.
Subclinical Hypothyroidism May Raise Premature Birth Risk by 50%
In 2016, the results of a large study conducted by a group of researchers from Boston University of Medicine was presented at the annual ATA (American Thyroid Association) meeting. Data was included from over 6,000 pregnant women who were seen at the hospital between 2007 and 2014. All of the women were over the age of 18 and pregnant with only one fetus. Women with a pre-existing thyroid condition (or on thyroid medication) were excluded. TSH (serum thyrotropin) values from the participants’ first visit were compared to TSH reference ranges established for pregnancy by the ATA in 2011. These values are as follows:
- 1st trimester – 0.1-2.5 mIU/L
- 2nd trimester – 0.2-3.0 mIU/L
- 3rd trimester – 0.3 – 3.0 mIU/L
Using these values, 6.3% of the women had elevated TSH. After adjusting for possible confounding factors, it was concluded that these women had a 52% increased risk for a premature birth, which was defined as less than 37 weeks. Dr. Lee, head researcher, reported that no association was found between subclinical hypothyroidism and other adverse pregnancy outcomes such as fetal loss, preterm labor, placental abruption, gestational diabetes, preeclampsia, etc. However, she also noted that the risk of miscarriage could not be adequately assessed since participants were screened at first visit and many miscarriages occur before the first prenatal appointment. This study makes the case for changes in screening guidelines, as universal screening is not currently recommended by the ATA.
More Evidence for Universal Screening
This study is not the only that has demonstrated this association. Another large study published in 2013, known as the “Generation R study,” made similar findings. In 2014, a study with 3,147 participants found an association between subclinical hypothyroidism and miscarriage, particularly in those who also had positive TPO-antibodies. Yet another smaller study found that treating subclinical hypothyroidism resulted in reduction of several pregnancy-related complications.
In this smaller study out of the Mayo Clinic, treatment with low-dose T4 (levothyroxine) in subclinical hypothyroid patients reduced the incidence of multiple pregnancy complications, including preterm delivery, low Apgar scores in the newborn, neonatal intensive care unit admission, low birth weight, and pregnancy loss. This reduction was seen in those with negative TPO-antibodies. However, there was an increased risk of gestational diabetes and premature rupture of membranes in the treatment group. None of these findings could be considered significant, though, due to the extremely small sample size. It was also noted that some patients’ TSH normalized on their own after the first trimester.
The Debate Continues
It is becoming clear that subclinical hypothyroidism can contribute to pregnancy complications and possibly neurocognitive development in offspring. However, since studies have shown mixed results in regards to treatment outcomes, the ATA is still reluctant to recommend universal screening for pregnant women. More research is needed. In the meantime, it is important that women of childbearing age are informed about this research, since it indicates that current recommendations for thyroid screening and treatment may be inadequate for the management of an optimal, healthy pregnancy. This way, patients can choose a healthcare practitioner that takes a more aggressive approach when it comes to testing for and addressing thyroid problems, if they so desire. Addressing nutritional factors that can contribute to low thyroid function is one conservative way that women who are considering getting pregnant can help ensure a healthy pregnancy.
1. Melville, N. Subclinical Hypothyroidism in Pregnancy: Link to Premature Delivery. Medscape. Available at: http://www.medscape.com/viewarticle/869324
2. Lee, SY et al. Association between maternal thyroid function in pregnant and obstetric and perinatal outcomes. American Thyroid Association Annual Meeting. September 21-25; 2016; Denver, CO.
3. 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. 27; 3: 315-89.