Thyroid problems are linked to an increased risk of miscarriage. Let’s take a look at the relationship between different thyroid conditions and miscarriage, and whether treatment can help reduce those risks and help you achieve your goal of having a healthy baby.
About your thyroid and miscarriage
Miscarriage, also known as spontaneous pregnancy loss, is typically defined as loss of pregnancy that takes place at less than 20 weeks of gestation, during your first or second trimesters of pregnancy.
In general, around 31 percent of all pregnancies end in miscarriage. Around 66 percent of those miscarriages take place before you even know you are pregnant. An estimated 10 to 15 percent of confirmed pregnancies end in miscarriage.
If you have a thyroid condition, your miscarriage risk is increased. Specifically, you face an increased risk if you fall into one of the following groups:
- You are hypothyroid and untreated, or you are insufficiently treated.
- You test positive for thyroid peroxidase (TPO) antibodies, which is evidence of autoimmune Hashimoto’s disease. Testing positive is also referred to as being TPOAb-positive.
- You are hyperthyroid and untreated, or insufficiently treated.
How does thyroid disease affect miscarriage risk?
Research has shown that endocrine problems — including thyroid disease — cause from 8 to 12 percent of all miscarriages.
Thyroid disease can increase the risk of miscarriage for a number of reasons:
- Thyroid disease is associated with ovulatory problems and luteal defects that are associated with miscarriage.
- Pregnancy greatly increases your need for thyroid hormone production, and thyroid dysfunction may prevent that increased production.
- Autoimmunity can affect your body's ability to make needed immunologic changes that support a pregnancy.
Hypothyroidism and miscarriage
According to the guidelines, if you are subclinically hypothyroid — defined as levels between 2.5 and the top-end of the reference range, which is usually around 4.5 to 5.0 mU/L — your TPO antibodies should be evaluated, and if they are positive, you should be treated with thyroid hormone replacement during pregnancy.
If you are TPOAb-negative but have a TSH above 10.0 mU/L, you should also be treated.
If you are hypothyroid and being treated with a thyroid hormone replacement drug like levothyroxine, your thyroid stimulating hormone (TSH) should be kept within the following target ranges:
- You should be maintained between 0.2 and 2.5 mU/L during your first trimester of pregnancy.
- You should be maintained between 0.3 and 3.0 mU/L during your second and third trimesters of pregnancy.
If you are on thyroid hormone replacement and your levels fall above the recommended 2.5 mU/L cutoff during the first trimester — a situation that affects more than 60 percent of pregnant women with hypothyroidism — you have a higher risk of miscarriage.
Hyperthyroidism and miscarriage
Overt hyperthyroidism — defined as a TSH below the reference range and elevated free T4 and free T3 levels — is associated with a significantly increased risk of miscarriage. If you are overtly hyperthyroid, you should be treated. The treatment of choice is the antithyroid drug PTU during the first trimester and methimazole during the subsequent trimesters of pregnancy.
Subclinical or borderline hyperthyroidism is associated with a slightly increased risk of miscarriage. Experts have not reached an agreement regarding the potential benefit of treating subclinical hyperthyroidism, and more research is needed.
Thyroid peroxidase (TPO) antibodies and miscarriage
An estimated 10 to 20 percent of women of reproductive age are TPOAb-positive. If you fall into this group, you have an increased risk of miscarriage. According to Alex Stagnaro-Green, M.D., a professor of medicine and obstetrics and gynecology and expert on managing thyroid dysfunction during pregnancy,
"If a woman has Hashimoto’s disease with antibodies, but with the thyroid hormone levels still being normal, that’s been associated with a two- to threefold increase of miscarriage."
Dr. Stagnaro-Green’s research found that women who are TPOAb-positive have double the risk of pregnancy loss — around 17 percent — compared to antibody-negative women, who have an 8.4 percent risk.
Other studies have shown that women who are TPOAb-positive have rates up to four times the rate of miscarriage in women who are antibody-negative.
If you are TPOAb-positive, you also have a higher risk of recurrent pregnancy loss — defined as two consecutive miscarriages, or three or more miscarriages over time.
Treatment for antibody-positive euthyroid women
According to the Guidelines, women who are TPOAb-positive, but have TSH levels within the reference range — a state known as “euthyroid” — have a significantly decreased rate of miscarriage when treated with thyroid hormone replacement medication. On the medication, miscarriage risk is reduced to 3.5 percent, compared to a 13.8 percent miscarriage rate in women who are TPOAb-positive but untreated.
One study from Belgium found that treating TPOAb-positive women who had a TSH above 1 mU/L reduced the rate of miscarriage from 16 percent to 0.
Some research studies have shown that intravenous immunoglobulin (IVIG) therapy may help prevent miscarriage in women who are antibody positive.
Assisted reproduction treatments (ART) and your thyroid
Assisted reproduction treatments (ART) such as intrauterine insemination (IUI) are used to overcome fertility. If you are subclinically hypothyroid, with a TSH above 2.5 mU/L, treatment with levothyroxine is associated with a higher pregnancy rate, and a reduction in the risk of miscarriage after pregnancy is achieved.
Research has also shown after in vitro fertilization (IVF), the pregnancy rate is also much higher — 22 percent — in women with a TSH below 2.5mU/L, compared to a 9 percent rate in women with a TSH above 2.5 mU/L.
The guidelines recommend that women with subclinical hypothyroidism who are undergoing IUI or IVF TSH be treated for a TSH above 2.5 mU/L.
If you’re treated for hypothyroidism, how can you lower your miscarriage risk?
If you are on thyroid hormone replacement medication and become pregnant, research shows that you, like half of all women in this situation, may need a significant increase in your medication dosage early in pregnancy to avoid complications, including miscarriage.
The guidelines recommend confirming the pregnancy as soon as possible. You should have a plan in place with your doctor to immediately increase your dosage of thyroid medication as soon as pregnancy is confirmed. You should also get retested soon after, so any further adjustments can be made to your dosage.
What should you do to reduce your risk of miscarriage?
- If you have a family history of thyroid problems or autoimmune disease, have a history of postpartum thyroiditis, a history of infertility, or previous miscarriage, you should have comprehensive thyroid screening before conception, during early pregnancy, and later in your pregnancy.
If you are diagnosed with hypothyroidism in pregnancy and your TSH is above 10.0, you should be treated with thyroid hormone replacement medication.
If you are hypothyroid and planning a pregnancy, make sure your TSH is in the target range. You should also have a plan in place with your doctor to increase your dose of thyroid hormone replacement medication as soon as your pregnancy is confirmed.
If you are hypothyroid and trying to get pregnant, plan to test for pregnancy as early as possible. Some pregnancy tests can detect pregnancy as early as seven days post-conception. As soon as your pregnancy is confirmed, start taking the agreed-upon increased dosage of thyroid hormone replacement medication.
If you are hypothyroid and pregnant, have your thyroid periodically tested throughout the pregnancy so that your medication dose can be adjusted as necessary, to stay within the target range.
If you are TPOAb-positive, but not being treated with medication for hypothyroidism, you should seriously consider thyroid hormone replacement treatment prior to or starting in early pregnancy.
If you are TPOAb-positive and have a history of recurrent pregnancy loss, you should be treated with thyroid hormone replacement treatment.
If you are undergoing ART procedures such as IUI or IVF and have subclinical hypothyroidism, or you are TPOAb-positive, you should seriously consider thyroid hormone replacement treatment prior to or starting in early pregnancy.
If you are overtly hyperthyroid during pregnancy, you should be treated with antithyroid drugs.
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