The American Thyroid Association has updated their guidelines for the
management of thyroid concerns in pregnancy, the details of which were
published in the journal Thyroid this past month. Many of us who work
in the field of fertility have long been aware of research suggesting
that ranges for TSH should be lower in pregnancy. It is truly great to
see that this has been formally recognized. Although these guidelines
were written for pregnancy, I also apply these to women with fertility
concerns who are preparing for pregnancy. Prevention is always best
when it comes to avoiding miscarriage.
Thyroid disease is very common in pregnancy. One of the reasons is thatone of the main thyroid hormones (free T4) decreases in pregnancy.Another
reason is that TBG (thyroxine binding globulin) increases during
pregnancy – TBG is a hormone that binds to the circulating thyroid
hormone, making it unavailable to act on receptors. This aggravates
cases of hypothyroidism by binding up the thyroid hormone that would
normally be available to work in the body.
hCG, the pregnancy hormone, has a profound effect on thyroid function too. Normally in pregnancy, hCG causes TSH (Thyroid stimulating hormone) to decrease.
TSH is produced by the pituitary gland and causes the thyroid gland to
make thyroid hormones. So in a healthy pregnancy, we expect a woman to
have a lower TSH than she usually would . This makes high levels of
TSH in pregnancy to be of even more concern and gives us a lower
"normal” reference range for pregnancy. When TSH levels are high, this
indicates that the thyroid function is low, as the pituitary is
attempting to stimulate more thyroid hormone production from the thyroid
Autoimmune thyroiditisis very common in
pregnancy as well. 1 in 10 pregnant women will develop antibodies to
the thyroid. Hypothyroidism develops in 16% of women with thyroid
antibodies. Thyroid antibodies are also associated with lower success
rates in IVF cycles, and increased miscarriage rates, even if there is
no hypothyroidism in the patient. Postpartum thyroiditis can occur in
around 50% of women who develop antibodies during pregnancy.
As you can see, thyroid conditions are a very common health problem,
and have great impact on pregnancies and fertility. Not only is thyroid
disease related to miscarriage, but subclinical hypothyroidism or
positive thyroid antibodies can impact the brain development of the
fetus and have been linked to poor intellectual development in the baby.
Hyperthyroidism is related to miscarriage and a host of problems such
as intrauterine growth restriction. So this is an issue we must take
The new guidelines suggest the following:
- Trimester specific tighter ranges for TSH:
|First trimester normal range
||0.1 to 2.5 mIU/L
||0.2 to 3.0 mIU/L
||0.3 to 3.0 mIU/L
- Women who are already receiving thyroid replacement therapy should increase their dose by 25% to 30% when they become pregnant.
- The total amount of iodine should be 250 ug from all dietary and supplemental sources.
- Monitoring is important to ensure that women with hypothyroidism or
subclinical hypothyroidism are not at risk. TSH should be measured once
every 4 weeks until 16 to 20 weeks’ gestation and at least once between
26 and 32 weeks’ gestation.
Basic nutrition for thyroid in pregnancy
Of note,although our salt is iodized in developed countries, there is a growing deficiency of iodine.This
is because the iodine in our salt supply is not well absorbed and
utilized. In pregnancy, there is a 50% increase in iodine requirements.
So, choose a prenatal with some iodine content, usually around
150-200ug per day. Do not exceed 500mcg total intake daily, as this can
pose an increased risk for hypothyroidism.
Selenium should be part of a prenatal vitamin.
Several studies have shown that selenium decreases the levels of
thyroid antibodies (anti thyroglobulin (anti TG) and anti
thyroidperosidase (anti TPO). A randomized controlled trial found
that supplementing with 200mcg of selenium daily during pregnancy and
the post partum period reduced the incidence of postpartum thyroiditis
in women who were positive for thyroid antibodies. 55 mcg should
suffice as prevention in healthy women.
Perinatal thyroid disease is very common and new research is rapidly
emerging on this topic. Thyroid disease affects fertility, pregnancy,
maternal and fetal health. Women should optimally try to establish
healthy thyroid function before conceiving: this way many concerns can
be prevented, and the health of both moms and babies will be protected.
- Selenium Supplementation in Patients with Autoimmune
Thyroiditis Decreases Thyroid Peroxidase Antibodies Concentrations JCEM
2002 87: 1687-1691
- Guidelines of the American Thyroid Association for the
Diagnosis and Management of Thyroid Disease During Pregnancy and
Postpartum. The American Thyroid Association Taskforce on Thyroid
Disease During Pregnancy and Postpartum. THYROID Volume 21, Number 10,
- Dietary Iodine: Why Are So Many Mothers Not Getting Enough? Renner R 2010. Environ Health Perspect 118:a438-a442.
prevalence of thyroid antibodies in euthyroid women with a history of
recurrent in-vitro fertilization failure Hum. Reprod. (2000) 15(3):