Women have many questions after a miscarriage. Unless the cause is discovered to be a genetic anomaly, infection, anatomical abnormality or advanced age, the answers are many times elusive. Our medical system does not fully investigate first trimester pregnancy loss unless it has happened two or sometimes three times. This can be devastating for people trying to conceive.
Understandably, women are looking for answers not only for emotional closure from a miscarriage, but ultimately to prevent it from recurring. One overlooked problem that could simply prevent many first-time and recurrent miscarriages needs to be given more attention: thyroid health.
What role does the thyroid play in conception?
Normally, the thyroid adapts readily to the increased energy demands of pregnancy. Dramatic rises in estrogen in turn change the levels of a carrier protein that functions to bind active blood thyroid hormone. Thyroid binding globulin (TBG) levels roughly double in early pregnancy, reducing the amount of thyroxine (T4) and triiodothyronine (T3) in the bloodstream.
T4 and its active metabolite T3 are crucial to optimal physiological functioning including energy and metabolism, heat generation, regular bowel movements and stable mood. In fact, these two hormones as prescription medications are themselves treatments for hormonal deficiencies. In pregnancy T4 and T3 not only serve this purpose for mother but also for her developing baby. To compensate for the reduction in free active hormone, the thyroid increases production of both T4 and T3. Eventually by week 20 all hormones level off and reach a steady state.
The pregnancy hormone hCG also plays a role in altering T4 and T3 levels. The higher the hCG the greater production of thyroid hormone. Similarly to estrogen and TBG this gradual rise plateaus once hCG peaks at the beginning of the second trimester. It is at this point that the fetus is able to produce significant amounts of thyroid hormone for his or herself, increasing gradually thereafter.
When women do not produce enough T4 a host of problems can surface. Given that up to 15 percent of the general population has some kind of issue with the thyroid (subclinical hypothyroidism, Hashimoto’s disease or euthyroid positive thyroid peroxidase antibodies), it’s surprising the thyroid isn’t given a brighter spotlight in family planning.
The potential problems faced by women with low thyroid function in pregnancy can be:
- Hypertension and preeclampsia
- Placental abruption
- Preterm delivery as early as 32 weeks
- Increased risk of Caesarean section
- Increased disease and death post delivery
- Postnatal psychological and cognitive impairment of baby
- Postpartum hemorrhage
Screening for thyroid disorders in pregnancy
Most professional societies and medical authorities have not adopted universal screening for thyroid disease in pregnancy. Instead, testing is restricted to women who have a family history of thyroid disease, type 1 diabetes, elevated thyroid antibodies (rarely tested), infertility, radiation to the head and neck, previous miscarriage, obesity, age over 30, and other symptoms suggestive of thyroid disease.
In fact, doctors may pass over symptoms mistaking them for common complaints of pregnancy such as constipation, fatigue and weight gain. These guidelines have met controversy in that they can miss 30 percent of women who will need thyroid support during pregnancy. This is especially true for women who go undiagnosed with positive thyroid antibodies because they are rarely tested. Many specialists and naturopathic physicians call for universal screening because, for them, the benefits outweigh the economic harms of testing for hypothyroidism across the board. Naturopathic physicians will privately test these hormones when public doctors will not.
TSH testing is not good enough
Thyroid stimulating hormone (TSH) does just that— stimulates the thyroid from the pituitary gland to produce T4 and T3. In non-pregnant populations the laboratory range of this hormone should be between 0.27 and 4.2. We know that hormones are in constant flux during pregnancy and so reference ranges also change to reflect a safer level for both mom and baby.
The serum range depends on the stage of pregnancy. Ideally TSH levels in the first trimester are from 0.1 to 2.5, in the second trimester 0.2 to 3.0 and the third trimester 0.3 to 3.0. T4 and T3 both increase to 1.5 times the non-pregnant values.
Evidence suggests these ranges are necessary to prevent miscarriage. In practice, these values are, unfortunately, habitually ignored. What’s even more disregarded than TSH is thyroid antibody testing, which also plays a significant role in maintaining pregnancy.
Why thyroid antibodies are so important
Women with no signs or symptoms of hypothyroidism, including normal TSH, T4 and T3, yet who have elevated thyroid antibodies (>35) are at a higher risk of miscarriage, preterm delivery and disease risk to the baby. When studied, the rate of spontaneous miscarriage increases just with a positive antibody test result and is up to three times as high as for those women without antibodies. When these women with positive antibodies were subsequently treated with thyroid hormone, the rate of miscarriage was reduced from 13.8 percent to 3.5 percent. The rate of preterm delivery also reduced from 22.4 percent to 8.2 percent. These percentage differences equate to important clinical decisions that need to be made between women wanting to conceive and their doctors.
In my practice I find it prudent and in the best interest of patients to proactively test for thyroid antibodies before conception efforts. Once presented with these facts patients find it an easy decision to want to assess their baseline thyroid health. This is especially true for women who have previously miscarried and have not had this done even approaching a second pregnancy. In other settings, like women undergoing assisted reproductive therapy (ART), the difference between miscarriage rates of women with positive antibodies to women with negative antibodies was 52 percent and 33 percent respectively. This further highlights how practical and sensible testing is for more vulnerable populations to miscarriage.
Other considerations like diet can also have significant impacts. Selenium is measured lower in women with recurrent pregnancy loss. Higher selenium intake is linked with lower thyroid antibodies.
What’s more, post pregnancy the differences between children born to mothers with positive antibodies score less than those born to mothers with negative antibodies on intellectual and motor development evaluations. Th differences at two years of age are 10 points lower on both assessments for children of antibody-positive mothers.
Subclinical hypothyroidism and pregnancy
Subclinical hypothyroidism is a symptomless picture with increased TSH but no change in T4. It is believed to be the time when thyroid disease is being caught incidentally earlier on blood tests even though no physical findings would suggest it otherwise. Risks are lower than for overt hypothyroidism, but they are ever present.
When studied, women who fall just outside of normal TSH reference ranges had almost double the rates of pregnancy loss. Given the potential harms, replacing thyroid hormone for women with subclinical hypothyroidism and positive antibodies is a meaningful preventive strategy.
Although evidence is not robust for the recommendations of screening and treatment, the physical and psychological ramifications of failed pregnancy can be distressing, damaging and long lasting. Advantages, in addition to successful pregnancies, are better cognitively developed infants, fewer complications of pregnancy and fewer cases of miscarriage-related psychological trauma.
For physicians working with women wanting to conceive, I believe thoroughly assessing thyroid status is warranted. For women wanting to become pregnant, I encourage greater awareness of thyroid health.
