It is estimated that 3-10% of women have hypothyroidism which, during pregnancy, has been associated with adverse outcomes of varying degrees, ranging from negative effects on fetal neuropsychological development to pregnancy loss.
Several studies have shown that maternal TSH levels above 2.5 mU/L, and especially above 4.5 mU/L, are linked to greater risk of miscarriage. In fact, one study found the pregnancy loss rate to be as high as 60-70% for women whose hypothyroidism is not adequately treated.
Since research has also shown that roughly 60% of hypothyroid women on levothyroxine have TSH levels greater than 2.5mU/L at their first prenatal appointment (the upper limit desired in the first trimester) and almost 40% have levels above 4.1mU/L, it is evident that this is an urgent concern in the field of endocrinology and obstetrics.
The availability of and increased need for thyroid hormone during pregnancy is crucial due to its role in supporting placental trophoblasts, which ensure proper delivery of blood flow and nutrients to the growing fetus. Women with normal thyroid function generally compensate for this increased need on their own, while those with hypothyroidism cannot – regardless of whether their TSH levels are in an acceptable range prior to pregnancy. It appears that this increased need for thyroid hormone plateaus by week 16, which means the first trimester is the most critical window of time for adequate thyroid treatment.
A 2010 study demonstrated that 92% of abnormal TSH levels could be identified if testing was performed every 4 weeks during early pregnancy. The researchers divided pregnant women into a treatment group or a control group. The treatment group increased their dose of levothyroxine by 2 tablets per week at the onset of pregnancy. This increase was shown to be very effective at reducing the risk of hypothyroidism during early pregnancy. Some of the women ended up with lower than desired TSH later in pregnancy, making the case for continual monitoring and a reduction of medication after the first trimester as needed.
These findings were corroborated recently when the results of a retrospective study on this topic was published in Clinical Endocrinology. Data was collected on ninety-three pregnant women between the ages of 18 and 45 who were seen at the Mayo Clinic between 2011 and 2013. All the women had a history of hypothyroidism, were being treated with levothyroxine thyroid hormone replacement, and had a TSH above 2.5mU/L in their first trimester. The women who did not have their medication increased during the first trimester had a pregnancy loss rate of 36% compared to a 2.4% pregnancy loss rate among those who were given the higher dose.
While some experts think levothyroxine should be increased by 30% during the first trimester, other findings, such as those of Verga et al, make the case for an even greater increase (45-70%).
Also up for debate is exactly when changes to treatment should be made. Some say thyroid function should even be optimized prior to pregnancy to ensure the best outcomes.
Pregnancy loss can be devastating for families. Therefore, this is an important topic that needs solutions faster than research can keep up. Thankfully, the recent progress on the topic is leading to changes. Since it often takes some time for the standard of care to be updated and implemented across the board among physicians, be sure to spread the word to friends and loved ones who plan to get pregnant – both those already diagnosed with hypothyroidism and those who suspect their thyroid function may be suboptimal.