Thyroid Hormones and Miscarriage Risk: Essential Insights for Expectant Mothers

By Rebecca Jaspan, MPH, RD, CDN, CDCES

Among a number of recognized causes of miscarriages, thyroid disease is a common one and often goes undiagnosed.  Thyroid hormones are essential for the growth and development of the fetus.  The mother supplies the fetus with thyroid hormones.  If she has hypothyroidism, or inadequate amounts of thyroid hormone, she cannot supply enough to the growing fetus and is therefore at risk for a miscarriage.¹

Thyroid Hormones and Their Role in Miscarriage

Thyroid hormone imbalances and disease are often seen in women of reproductive age.  Any thyroid dysfunction causes disturbances in human reproductive physiology and may reduce the likelihood of pregnancy and adversely affects pregnancy outcomes.  It is estimated that 8-12% of all pregnancy losses are due to endocrine factors.  Thyroid disease is present in 4% of women of reproductive age and up to 15% of women are at risk for miscarriages because they may be thyroid-antibody positive.²

What are the hormone levels to look for?

Thyroid-stimulating hormone, or TSH, plays a role in miscarriage.  The risk of miscarriage increases as TSH increases.  A normal TSH level is .2-2.5mU/L.  One study that looked at women with hypothyroidism found that women with TSH of 4.5-10mU/L had 1.8 times more chance of miscarriage compared to women with normal TSH levels during early pregnancy.  It was also noted that women with TSH levels of 2.51-4.5mU/L did not have an increased risk of miscarriage.³

While there is not yet enough evidence for direct causality and more research needs to be done in this area, it is known that thyroid disease is related to unexplained infertility and implantation failure.  Existing and poorly controlled hyper- or hypothyroidism are also associated with infertility and miscarriage.  Even mild hypothyroidism shows increased rates of miscarriage and may also have adverse effects on the cognitive development of the fetus.⁴  Additionally, excess thyroid hormone increases the risk of miscarriage.⁵

When a woman becomes pregnant, significant changes occur in the thyroid gland including changes in iodine metabolism, serum thyroid binding proteins, and potential development of goiter, which is an irregular growth of the thyroid, in the mother.⁴  There are also immunologic changes that occur during pregnancy.  Thyroid peroxidase antibodies may rise and research shows that they are present in 10% of women at 14 weeks gestation.  This is associated with miscarriage, increased incidence of gestational thyroid dysfunction, and predisposition for postpartum thyroid dysfunction.⁶

Screening for Thyroid Problems

At the current time, there are no recommendations for routine screening of thyroid abnormalities. However, if you are at risk for thyroid disease, it is recommended to speak with your doctor about checking your thyroid function and testing your blood for thyroid hormone and antibody levels.  Additionally, if you feel you have symptoms of hyper- or hypothyroidism, ask your doctor about testing.  If you have been treated for hyperthyroidism in the past with radioactive iodine treatment or surgery, make sure to alert your doctor to this as well, as your body will still produce antibodies that could affect the baby.

Treatment of Thyroid Problems and Miscarriage Prevention

Treatment for thyroid conditions is typically correcting for over or underproduction of thyroid hormones with medication.  Levothyroxine is used for hypothyroidism to replace thyroid hormones and methimazole is used for hyperthyroidism to reduce excess hormone levels.  These medications are considered safe to take throughout pregnancy.  Your thyroid hormones will be monitored throughout pregnancy as changes in hormone production often cause women to no longer need medication by their third trimester.  There are also limited studies showing that giving the hormone thyroxine may be effective at preventing miscarriage when given in the early stages of pregnancy.²

While it is difficult to know if a thyroid problem caused a previous miscarriage, if you are aware of a thyroid problem, it is a good idea to explore the condition with your doctor and manage it well before getting pregnant again.

References:

1. Rao VR, Lakshmi A, Sadhnani MD. Prevalence of hypothyroidism in recurrent pregnancy loss in first trimester. Indian J Med Sci 2008;62:357-61.
2. Sarkar D. Recurrent pregnancy loss in patients with thyroid dysfunction. Indian J Endocrinol Metab. 2012;16(Suppl 2):S350-S351. doi:10.4103/2230-8210.104088
3. Vol 7 Issue 12 p.3. American Thyroid Association.
4. Kennedy RL, Malabu UH, Jarrod G, Nigam P, Kannan K, Rane A. Thyroid function and pregnancy: Before, during and beyond. J Obstet Gynaecol. 2010;30:774–83.
5. https://www.uptodate.com/contents/recurrent-pregnancy-loss-definition-and-etiology
6. Abbassi-Ghanavati M. Thyroid autoantibodies and pregnancy outcomes. Clin Obstet Gynecol. 2011;54:499–505.